We booked w/AirTran in April for our Sept. flight. They have changed our flight times a half dozen times, by 2 minutes or so each time. After the last time I called and asked to change flights b/c I didn't like the new time. Not only did they let me change with no fee, because the changes were their fault, they let me change to a direct flight rather than a layover!
The helpful operator told me they would find me another flight the day of, before, or after. So yes, I would hope they change your flight time then you can change flights with no fee.