The easiest way to transfer a ticket to another event is by cancelling the original ticket and issuing a free ticket to the other event.
On the order page, find the ticket and click on "...":
Choose "Cancel ticket" so that the order will remain completed but the ticket will be cancelled. This increases the original event availability and releases the seat if needed.
Now, go to your new event, open Orders from the left menu and choose "Send free tickets".