diff --git a/__test__/components/EventModifications.test.tsx b/__test__/components/EventModifications.test.tsx
index 185f698..260603a 100644
--- a/__test__/components/EventModifications.test.tsx
+++ b/__test__/components/EventModifications.test.tsx
@@ -2,10 +2,22 @@ import { CalendarApi } from "@fullcalendar/core";
 import { jest } from "@jest/globals";
 import { ThunkDispatch } from "@reduxjs/toolkit";
 import "@testing-library/jest-dom";
-import { act, screen, waitFor, within } from "@testing-library/react";
+import {
+  act,
+  fireEvent,
+  screen,
+  waitFor,
+  within,
+} from "@testing-library/react";
 import * as appHooks from "../../src/app/hooks";
+import * as eventThunks from "../../src/features/Calendars/CalendarSlice";
 import CalendarApp from "../../src/components/Calendar/Calendar";
+import EventUpdateModal from "../../src/features/Events/EventUpdateModal";
 import { renderWithProviders } from "../utils/Renderwithproviders";
+import {
+  createEventHandlers,
+  EventHandlersProps,
+} from "../../src/components/Calendar/handlers/eventHandlers";
 
 describe("CalendarApp integration", () => {
   const today = new Date();
@@ -260,4 +272,283 @@ describe("CalendarApp integration", () => {
 
     expect(screen.getByText("Public Event")).toBeInTheDocument();
   });
+  describe("BUGFIX", () => {
+    const preloadedState = {
+      user: {
+        userData: {
+          sub: "test",
+          email: "test@test.com",
+          sid: "mockSid",
+          openpaasId: "667037022b752d0026472254",
+        },
+        tokens: {
+          accessToken: "token",
+        },
+      },
+      calendars: {
+        list: {
+          "667037022b752d0026472254/cal1": {
+            name: "Calendar 1",
+            id: "667037022b752d0026472254/cal1",
+            color: { light: "#FFFFFF", dark: "#000000" },
+            ownerEmails: ["alice@example.com"],
+            events: {
+              event1: {
+                id: "event1",
+                calId: "667037022b752d0026472254/cal1",
+                uid: "event1",
+                title: "Original Event",
+                start: new Date("2025-11-14T10:31:00.000Z").toISOString(),
+                end: new Date("2025-11-14T11:31:00.000Z").toISOString(),
+                class: "PUBLIC",
+                partstat: "ACCEPTED",
+                organizer: {
+                  cn: "Alice",
+                  cal_address: "alice@example.com",
+                },
+                attendee: [
+                  {
+                    cn: "Alice",
+                    partstat: "ACCEPTED",
+                    rsvp: "TRUE",
+                    role: "CHAIR",
+                    cutype: "INDIVIDUAL",
+                    cal_address: "alice@example.com",
+                  },
+                  {
+                    cn: "Bob",
+                    partstat: "ACCEPTED",
+                    rsvp: "TRUE",
+                    role: "CHAIR",
+                    cutype: "INDIVIDUAL",
+                    cal_address: "bob@example.com",
+                  },
+                ],
+              },
+            },
+          },
+        },
+        pending: false,
+      },
+    };
+
+    it("keeps all attendees event participation on title update", async () => {
+      const updateSpy = jest.spyOn(eventThunks, "putEventAsync");
+      const onClose = jest.fn();
+      renderWithProviders(
+        <EventUpdateModal
+          eventId="event1"
+          calId="667037022b752d0026472254/cal1"
+          open={true}
+          onClose={onClose}
+          typeOfAction="solo"
+        />,
+        preloadedState
+      );
+
+      const titleInput = await screen.findByDisplayValue("Original Event");
+
+      await act(async () => {
+        fireEvent.change(titleInput, "Updated Event");
+      });
+
+      const saveButton = screen.getByRole("button", { name: /save/i });
+      await act(async () => {
+        saveButton.click();
+      });
+
+      await waitFor(() => expect(updateSpy).toHaveBeenCalled());
+
+      const dispatchedCalls = updateSpy.mock.calls;
+      expect(dispatchedCalls.length).toBeGreaterThan(0);
+      const updatedEvent = dispatchedCalls[0][0].newEvent;
+
+      // Ensure organizer attendee info is preserved
+      const organizerAttendee = updatedEvent?.attendee?.find(
+        (a: any) => a.cal_address === "alice@example.com"
+      );
+
+      expect(organizerAttendee).toBeTruthy();
+      expect(organizerAttendee?.partstat).toBe("ACCEPTED");
+      expect(organizerAttendee?.role).toBe("CHAIR");
+
+      // Ensure normal attendee info is preserved too
+      const normalAttendee = updatedEvent?.attendee?.find(
+        (a: any) => a.cal_address === "bob@example.com"
+      );
+
+      expect(normalAttendee).toBeTruthy();
+      expect(normalAttendee?.partstat).toBe("ACCEPTED");
+      expect(normalAttendee?.role).toBe("CHAIR"); //will need to be changed as not the right role
+    });
+
+    it("changes normal attendee to need action on time update and no organizer changes", async () => {
+      const updateSpy = jest.spyOn(eventThunks, "putEventAsync");
+      const onClose = jest.fn();
+      renderWithProviders(
+        <EventUpdateModal
+          eventId="event1"
+          calId="667037022b752d0026472254/cal1"
+          open={true}
+          onClose={onClose}
+          typeOfAction="solo"
+        />,
+        preloadedState
+      );
+
+      const startDateInput = await screen.findByTestId("start-time-input");
+
+      await act(async () => {
+        fireEvent.change(startDateInput, {
+          target: { value: "08:00" },
+        });
+      });
+
+      const saveButton = screen.getByRole("button", { name: /save/i });
+      await act(async () => {
+        saveButton.click();
+      });
+
+      await waitFor(() => expect(updateSpy).toHaveBeenCalled());
+
+      const dispatchedCalls = updateSpy.mock.calls;
+      expect(dispatchedCalls.length).toBeGreaterThan(0);
+      const updatedEvent = dispatchedCalls[0][0].newEvent;
+
+      // Ensure organizer attendee info is preserved
+      const organizerAttendee = updatedEvent?.attendee?.find(
+        (a: any) => a.cal_address === "alice@example.com"
+      );
+
+      expect(organizerAttendee).toBeTruthy();
+      expect(organizerAttendee?.partstat).toBe("ACCEPTED");
+      expect(organizerAttendee?.role).toBe("CHAIR");
+
+      // Ensure normal attendee partstat is updated
+      const normalAttendee = updatedEvent?.attendee?.find(
+        (a: any) => a.cal_address === "bob@example.com"
+      );
+
+      expect(normalAttendee).toBeTruthy();
+      expect(normalAttendee?.partstat).toBe("NEEDS-ACTION");
+      expect(normalAttendee?.role).toBe("CHAIR"); //will need to be changed as not the right role
+    });
+    it("update event attendees on drag", async () => {
+      const mockDispatch = jest.fn();
+      const updateSpy = jest.spyOn(eventThunks, "putEventAsync");
+
+      const eventHandlers = createEventHandlers({
+        setSelectedRange: jest.fn(),
+        setOpenEventModal: jest.fn(),
+        setTempEvent: jest.fn(),
+        setOpenEventDisplay: jest.fn(),
+        dispatch: mockDispatch,
+        calendarRange: { start: new Date(), end: new Date() },
+        setEventDisplayedId: jest.fn(),
+        setEventDisplayedCalId: jest.fn(),
+        setEventDisplayedTemp: jest.fn(),
+        calendars: preloadedState.calendars.list,
+        setSelectedEvent: jest.fn(),
+        setAfterChoiceFunc: jest.fn(),
+        setOpenEditModePopup: jest.fn(),
+      } as unknown as EventHandlersProps);
+
+      const mockArg = {
+        event: {
+          _def: {
+            extendedProps: {
+              uid: "event1",
+              calId: "667037022b752d0026472254/cal1",
+            },
+          },
+        },
+        // drag event → move by 1 day
+        delta: { years: 0, months: 0, days: 1, milliseconds: 0 },
+      };
+
+      renderCalendar();
+      eventHandlers.handleEventDrop(mockArg);
+
+      expect(updateSpy).toHaveBeenCalled();
+
+      // Extract the dispatched update event
+      const dispatched = updateSpy.mock.calls[0];
+
+      expect(dispatched).toBeTruthy();
+
+      const updatePayload = dispatched[0];
+      const updatedEvent = updatePayload.newEvent;
+
+      // Organizer should remain unchanged
+      const organizer = updatedEvent.attendee.find(
+        (a: any) => a.cal_address === "alice@example.com"
+      );
+      expect(organizer?.partstat).toBe("ACCEPTED");
+
+      // Normal attendee must become NEEDS-ACTION
+      const normal = updatedEvent.attendee.find(
+        (a: any) => a.cal_address === "bob@example.com"
+      );
+      expect(normal?.partstat).toBe("NEEDS-ACTION");
+    });
+
+    it("update event attendees on resize", async () => {
+      const mockDispatch = jest.fn();
+      const updateSpy = jest.spyOn(eventThunks, "putEventAsync");
+
+      const eventHandlers = createEventHandlers({
+        setSelectedRange: jest.fn(),
+        setOpenEventModal: jest.fn(),
+        setTempEvent: jest.fn(),
+        setOpenEventDisplay: jest.fn(),
+        dispatch: mockDispatch,
+        calendarRange: { start: new Date(), end: new Date() },
+        setEventDisplayedId: jest.fn(),
+        setEventDisplayedCalId: jest.fn(),
+        setEventDisplayedTemp: jest.fn(),
+        calendars: preloadedState.calendars.list,
+        setSelectedEvent: jest.fn(),
+        setAfterChoiceFunc: jest.fn(),
+        setOpenEditModePopup: jest.fn(),
+      } as unknown as EventHandlersProps);
+
+      const mockArg = {
+        event: {
+          _def: {
+            extendedProps: {
+              uid: "event1",
+              calId: "667037022b752d0026472254/cal1",
+            },
+          },
+        },
+        startDelta: { years: 0, months: 0, days: 0, milliseconds: 0 },
+        endDelta: { years: 0, months: 0, days: 0, milliseconds: 3600000 }, // 1 hour
+      };
+
+      renderCalendar();
+      eventHandlers.handleEventResize(mockArg);
+
+      expect(updateSpy).toHaveBeenCalled();
+
+      // Extract the dispatched update event
+      const dispatched = updateSpy.mock.calls[0];
+
+      expect(dispatched).toBeTruthy();
+
+      const updatePayload = dispatched[0];
+      const updatedEvent = updatePayload.newEvent;
+
+      // Organizer should remain unchanged
+      const organizer = updatedEvent.attendee.find(
+        (a: any) => a.cal_address === "alice@example.com"
+      );
+      expect(organizer?.partstat).toBe("ACCEPTED");
+
+      // Normal attendee must become NEEDS-ACTION
+      const normal = updatedEvent.attendee.find(
+        (a: any) => a.cal_address === "bob@example.com"
+      );
+      expect(normal?.partstat).toBe("NEEDS-ACTION");
+    });
+  });
 });
diff --git a/src/components/Calendar/handlers/eventHandlers.ts b/src/components/Calendar/handlers/eventHandlers.ts
index 9bd6c9a..fd71445 100644
--- a/src/components/Calendar/handlers/eventHandlers.ts
+++ b/src/components/Calendar/handlers/eventHandlers.ts
@@ -17,6 +17,7 @@ import { refreshCalendars } from "../../Event/utils/eventUtils";
 import { updateTempCalendar } from "../utils/calendarUtils";
 import { User } from "../../Attendees/PeopleSearch";
 import { formatLocalDateTime } from "../../Event/utils/dateTimeFormatters";
+import { userAttendee } from "../../../features/User/userDataTypes";
 
 export interface EventHandlersProps {
   setSelectedRange: (range: DateSelectArg | null) => void;
@@ -171,11 +172,14 @@ export const createEventHandlers = (props: EventHandlersProps) => {
     const computedNewStart = new Date(originalStart.getTime() + totalDeltaMs);
     const originalEnd = new Date(event.end ?? "");
     const computedNewEnd = new Date(originalEnd.getTime() + totalDeltaMs);
-    const newEvent = {
-      ...event,
-      start: computedNewStart.toISOString(),
-      end: computedNewEnd.toISOString(),
-    } as CalendarEvent;
+    const newEvent = updateAttendeesAfterTimeChange(
+      {
+        ...event,
+        start: computedNewStart.toISOString(),
+        end: computedNewEnd.toISOString(),
+      } as CalendarEvent,
+      true
+    );
 
     if (isRecurring) {
       setSelectedEvent(event);
@@ -245,11 +249,14 @@ export const createEventHandlers = (props: EventHandlersProps) => {
     const computedNewEnd = new Date(
       originalEnd.getTime() + getDeltaInMilliseconds(arg.endDelta)
     );
-    const newEvent = {
-      ...event,
-      start: computedNewStart.toISOString(),
-      end: computedNewEnd.toISOString(),
-    } as CalendarEvent;
+    const newEvent = updateAttendeesAfterTimeChange(
+      {
+        ...event,
+        start: computedNewStart.toISOString(),
+        end: computedNewEnd.toISOString(),
+      } as CalendarEvent,
+      true
+    );
 
     if (isRecurring) {
       setSelectedEvent(event);
@@ -308,3 +315,55 @@ export const createEventHandlers = (props: EventHandlersProps) => {
     handleEventResize,
   };
 };
+
+export const updateAttendeesAfterTimeChange = (
+  event: CalendarEvent,
+  timeChanged?: boolean,
+  attendees?: userAttendee[]
+): CalendarEvent => {
+  const { attendee, organizer } = event;
+  if (!attendee || !organizer) return event;
+
+  const organizerAddr = organizer.cal_address;
+
+  const markNeedsAction = (att: userAttendee): userAttendee => ({
+    ...att,
+    partstat: "NEEDS-ACTION",
+    rsvp: "TRUE",
+  });
+
+  const getExistingOrDefault = (addr: string, fallback: userAttendee) =>
+    attendee.find((a) => a?.cal_address === addr) ?? fallback;
+
+  if (attendees) {
+    const updatedAttendees = attendees.map((att) => {
+      const existing = getExistingOrDefault(
+        att.cal_address,
+        markNeedsAction(att)
+      );
+      return timeChanged ? markNeedsAction(existing) : existing;
+    });
+
+    const organizerEntry = getExistingOrDefault(organizerAddr, {
+      ...organizer,
+      role: "CHAIR",
+      cutype: "INDIVIDUAL",
+      partstat: "NEEDS-ACTION",
+      rsvp: "TRUE",
+    });
+
+    return {
+      ...event,
+      attendee: [...updatedAttendees, organizerEntry],
+    };
+  }
+  const updatedAttendees = attendee.map((att) => {
+    if (att.cal_address === organizerAddr) return att;
+    return timeChanged ? markNeedsAction(att) : att;
+  });
+
+  return {
+    ...event,
+    attendee: updatedAttendees,
+  };
+};
diff --git a/src/features/Events/EventUpdateModal.tsx b/src/features/Events/EventUpdateModal.tsx
index 3524841..a12c64c 100644
--- a/src/features/Events/EventUpdateModal.tsx
+++ b/src/features/Events/EventUpdateModal.tsx
@@ -29,6 +29,7 @@ import {
 } from "./eventUtils";
 import { updateTempCalendar } from "../../components/Calendar/utils/calendarUtils";
 import { useI18n } from "cozy-ui/transpiled/react/providers/I18n";
+import { updateAttendeesAfterTimeChange } from "../../components/Calendar/handlers/eventHandlers";
 
 const showErrorNotification = (message: string) => {
   console.error(`[ERROR] ${message}`);
@@ -407,7 +408,14 @@ function EventUpdateModal({
       }
     }
 
+    const eventStartChanged =
+      new Date(event.start).getTime() !== new Date(startDate).getTime();
+    const eventEndChanged =
+      new Date(event?.end ?? "").getTime() !== new Date(endDate).getTime();
+    const timeChanged = eventStartChanged || eventEndChanged;
+
     const newEvent: CalendarEvent = {
+      ...updateAttendeesAfterTimeChange(event, timeChanged, attendees),
       calId: newCalId || calId,
       title,
       URL: event.URL ?? `/calendars/${newCalId || calId}/${event.uid}.ics`,
@@ -421,9 +429,6 @@ function EventUpdateModal({
       class: eventClass,
       organizer: organizer,
       timezone,
-      attendee: organizer
-        ? [organizer as userAttendee, ...attendees]
-        : attendees,
       transp: busy,
       color: targetCalendar?.color,
       alarm: { trigger: alarm, action: "EMAIL" },