FAQ Q369: How can I assign an existing reservation to a different customer (user id)?

Go to the reservation details page of the reservation in question. Then click on the button assign to different customer and type the customer's name, email address or user id. In the search results you'll see the matches; choose the correct user from this list.

If you have a number of reservations that you wish to move from one user to another, go to a page which shows the listing of these reservations (it could be on the user details page, or use the green reservation search button in the top-right corner of each backend page in order to find the target reservations. Then click on the button Bulk manage items on the page with the listing, select the reservations (or click select all to select all reservations from the list) and then click on assign to different customer, similar to how this is done for a single reservation.