Not eligible on TATL flights. You can always ask, but generally international does allow the same conveniences we take for granted sometimes travelling domestically.
http://www.usairways.com/en-US/trave...s/default.html
Move up program
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). You can only make day-of-departure changes at the airport (and not by calling Reservations).