I forgot to log out of a different account in Gmail and I made a bunch of changes to the calendar. Can I copy it to the other account?
Open the event. Under "More Actions" is an option: Change owner
Click that and put the email address of the other account in as the new owner. That account will receive a notification that you're trying to transfer ownership of the event.
Log in to the other account and click the confirmation link in the email you receive. That will bring you to the event edit screen and you can make whatever changes you need and save the event.
From my tests the event will stay on the original calendar unless you specifically remove it.