What happens if my customer wants to change the dates on a hotel booking after it is already confirmed?

Usually no, not as a straight edit to the existing booking. Most hotel suppliers do not let you swap the check in and check out dates on a confirmed reservation, because the room, the rate and the availability are all tied together at the moment of booking. The normal route is to cancel the original booking and make a new one for the new dates, and your portal is built to let you do both from the booking record.

Whether that costs your customer anything depends entirely on the rate they booked. If it was a refundable rate and you are still inside the free cancellation window, you cancel at no charge and rebook. If it was a non refundable rate, the supplier will usually keep the money even though your customer is only moving the dates, so you need to check the cancellation terms on the booking before you tell the customer anything. The new dates are also priced at whatever is available today, which can be higher or lower than what they first paid.

A few suppliers do allow some changes to be sent through, and some will accept a date change as a request rather than an instant action. That varies from supplier to supplier and is not something your portal can force, so the honest answer is that it has to be checked case by case with the supplier holding the booking.

Before you promise a customer anything, open the booking in your portal and read the cancellation terms on that specific rate, then confirm with your supplier whether they will take a change request for it.