I've never been able to change a booking online, but I've been able to change them on the phone. Either by calling the hotel directly or by calling central reservations--I find the reservation agents at my local Canada call center are pretty good, but I don't have experience with the overseas call centres. However, most of the time, I just go online and make a new booking, and then cancel the old one. That works for me, because I rarely ever book anything prepaid.