The situation may arise when you need to move a booking from one user to another. This is a quick and easy step in the 'edit booking' modal.
Open the booking-edit modal, click on the blue pencil icon and select the new holder.
Both the new and previous holders will be informed of the change (assuming user notifications are enabled in your venue settings).
A pro tip: If you don't yet have a user's details (full name, email address) but would like to add their booking into the system immediately to secure the time and space, create the booking under the 'casual-user' holder and add the user's name into the notes field. Then when you have their details, change the booking holder from 'casual-user' to the newly added holder.