Unfortunately, I've had a few occasions where I've had to cancel. For US Air, you have to go through the re-issue department to book the new flights. I've had to reschedule Air Tran as well, but for some reason that went through Travelocity. For the latter, I had to actually send in my boarding passes to get the re-issue. The charge is $50 and the difference in fare price. I believe it is up to $75 per ticket with US Air. If you can find a cheaper fare than the tickets you cancelled, then you will reduce the out-of-pocket expense for the change fee.
For Air Tran, the new fare had to be booked one year from the original booking date . In '98, USAir had a two-year window...don't know if that's changed. Don't wait until the last minute to do the change, allow yourself at least one month before expiration in case you have to send in boarding passes, etc. I rebooked mine a few days before 9/11. If I had waited any longer, it's doubtful I could have gotten through and gotten the rebooking with all the chaos that occured at that time.
Another thing I do before doing the reissue is to look up the flight and price it online. Although you may not be able to get the online price, the fare you do get will usually be within a few dollars. I'd rather be armed with information as to the flight I want and the fare it's currently going for than be at the mercy of the airline reservationist.
Hope this helps!
Sharon