Actually, you can sometimes change it with priceline. They won't do it if your plans change and you just want a new date, but if you bid on the wrong dates because you just had it wrong in your head or something, what they will do is allow you to cancel the original reservation for $25/night (or something like that), provided that you rebid for the new dates. Then, if you win on the right dates, you can get some portion of that cancellation fee back. I can't remember the exact figures and there are a number of restrictions to make sure people can't cancel just for fun, and it's probably harder to do if you catch your mistake several weeks after your booking rather than immediately, but priceline does have some limited means of dealing with this sort of situation.