There is currently no way to do this in bulk. Each activity will need to be manually unsynced from O365 before the organizer can be changed. Then it can be resynced.
Articles in this section
- Why Does it Say Pending Outlook Sync for a User on this Activity?
- How can I change the organizer on activities where the current organizer is no longer active with the firm?
- Activity Organizer
- Due Date & End Date for Tasks and Appointments
- Calendar Filters
- Modifying the Appearance of Tasks and Appointments on the Calendar
- Calendar Overview