Generally speaking if the airline changes the flight times by more than 60 minutes they will let you rebook to available seats whether or not your fare is available on the new flight.
Different airlines use different tactics to avoid telling you this and US Air seems to be one of the worst.....in our experience we have always had to argue and track down supervisors and follow up, etc, etc....in other words they make it as difficult for you as possible, but they can and WILL change your flight or refund your full purchase price. A change of 8 hours is huge, so don't be shy about asking for what you want.
Lots of other airlines (Delta, Continental, American, etc) seem a lot more co=operative....their phone agents can make the changes and don't seem reluctant to do so...I have never had SW change a flight on me, so don't know how they would handle it. US Air is also awful about notifying you of flight time changes....you had better check yourself, because the chances of them ever emailing or calling you are slim to none. Other airlines seem more conscientious about that too!!