if I use my $20 credit on a late flight and then an eariler flight come up can I cancell and get the original ($20)credit back and the new credit( the difference) and change my flight.
assuming SWA?
if so, you would prob want to leave the "old" credit applied to your new flight. then take a totally new credit (from the new monies used) for the difference.
this way, the credit would have a later expiration.