Can I change the dates on a hotel booking after my customer has already booked?

Sometimes, and it depends on the supplier rather than on your portal. A few hotel suppliers accept a change request through the API, so your staff can move the dates or swap a guest name from inside the booking screen and get an answer back within a minute. Many others do not offer that at all, and the only way to move the dates is to cancel the booking and make a new one at whatever the rate is on the day you rebook.

That second route is where the money is lost, so it is worth understanding before you promise anything. If the booking is still inside its free cancellation window, cancelling and rebooking usually costs nothing beyond the difference in price, which can go up or down. If the window has passed, or the rate was a non refundable one, the hotel is entitled to keep the charge and your customer pays twice. Your portal can show the deadline and the rate type on the booking, so your staff can see which situation they are in before they touch anything.

Guest names and small details are often easier than dates. Correcting a spelling, adding a late arrival note or changing the lead guest can frequently be passed to the hotel as a request, although a request is not a guarantee and the property can decline it.

Ask each of your suppliers which changes they accept through the API and which need an email to their support desk, then write that into your own booking process so your staff quote the right answer to the customer the first time.