Can I change the guest name on a hotel booking after my customer has already paid?

Yes in many cases, though not always, and the answer comes from the hotel and the supplier behind the rate rather than from your portal. A hotel booking API can send a name change on a confirmed booking, and plenty of hotels accept it because the room sits under your agency rather than under the traveller. Others treat the room as belonging to the named guest and will not swap it, in which case cancelling and rebooking is the only route. You find out which of the two you are dealing with when the request goes through.

The rate your customer bought matters most here. On flexible rates, a refused name change is rarely a problem, because you can usually cancel within the free period and book again for the correct guest. Non refundable rates are far tighter, and even when the hotel agrees it may treat the amendment as a new booking and charge for it, or simply decline. We cannot tell you the outcome before the supplier replies.

A small correction is not the same thing as a change of guest. Fixing a spelling, adding a middle name or putting the first and last name the right way round is usually straightforward, and is often handled by a quick message to the supplier instead of a formal amendment. Swapping one traveller for a completely different person is the request that tends to get turned down.

Before you promise your customer anything, open the booking in your portal and read the amendment and cancellation terms held against that rate, then send the request as early as you can, since hotels help far more readily while there is still time to resell the room.