Don't call Priceline, call the airline your ticket is on.

I got a Priceline ticket on DL, and they had no problem letting me fly stand by for an earlier flight, but that will vary by airline.

In general I don't think most airlines have problems with letting folks fly stand by for an earlier flight (except SW, where you have to pay full fare to stand by) ... If you think about it they're better of trying to get you there earlier than risking a later flight ... no one ever knows what will happen with weather/delays/etc.