Change the Organizer from Event Details
To move an event from one organizer to another, open the event in your admin console and click Change Organizer from the Event Details tab. Select which organizer should be associated with the event from the pop-up.
If the organizer does not appear in the pop-up, you need to add or change the role of one of the organizer admins to "admin" in Settings > Manage Team and have them move the event to the proper organization.
Note: If an event has been published, you won't be able to move them to another organizer. You can unpublish the event first, then move it.