From US website, Move Up Program
Move-Up Program
The new US Airways* understands that travelers need flexibility in their schedule. That's why we allow you to "move up" to any earlier flight on the same day of your originally scheduled departure time at the airport (with the exception of flights to Hawaii and Europe). Please note: Day-of-departure changes may only be made at the airport and not through our Reservations centers.
If there is an open seat available on any earlier flight that departs on the same day as your originally scheduled departure, you may change to that flight and we will automatically confirm your reservation for $25 for flights within the 48 contiguous United States and $50 for flights to Latin America, the Caribbean, Canada and Alaska. This lower confirmation fee replaces the usual $100 change fee, plus any differences between your old fare and new fare, for non-refundable tickets.
If there is NOT an open seat available on an earlier flight that departs on the same day as your originally scheduled departure, you may stand by for that flight at no charge. Standing by at no charge is not permitted if we can confirm that there is an open seat available for that flight. Seats on completely booked flights may become available if passengers with confirmed reservations don't show up for the flight. Please note: Unlike a confirmed reservation, flying standby does not guarantee that you will get on the particular flight that you want and may involve waiting for two or more flights before a seat is available to you.
*Until such time that US Airways and America West are operating with one Reservations system, stand-by or move-up will only be permitted on the originally booked carrier. If your original flight is operated by America West, you will be able to stand-by or move-up on America West operated flights only and if your original booked flight is on US Airways, you will be able to stand-by or move-up on US Airways operated flights only. Please pardon our dust while we work on integrating our computer systems. We do apologize for this inconvenience while we become one airline.