The administration and maintenance of Microsoft Exchange Server to ensure secure, reliable, and efficient email and collaboration services across an organization.
There is currently no way to change the organizer. Its been a long time ask, hopefully one day:
https://www.slipstick.com/outlook/change-meeting-organizer-outlook-meetings/
The meetings will need to be cancelled and recreated by the new owner
If the original owners mailbox is still around and enabled, the admin can cancel the meetings using powershell
https://learn.microsoft.com/en-us/powershell/module/exchange/remove-calendarevents?view=exchange-ps