If you find that a registrant needs to move their event registration from one event to another, here's the process to make that happen.
1) Log in to Salesforce
2) Go to the Opportunity record related to the event registration order
3) Update the Primary Campaign Source field to be that of the new event
4) Save the updated Opportunity record
5) On the Opportunity record page, scroll down to the Tickets related list, and edit the Campaign field for each Ticket to be that of the new event Campaign record, saving each Ticket
That's it! Happy event managing!