Call SWA. They will give you a credit on account if they can book the seats at the cheaper fare. You must use this credit within one year of the date you originally booked.
FYI - we had a credit with SWA - we chose not to fly a week after the Sept. 11th tragedies. We could not use the credit within a year, so I called SWA. They explained how someone else could use the credit toward a ticket, and the information that person would need to prove they were allowed to use it. In this case, my brother used the credit for a trip he had to make, so the money did not go to waste.
Good luck!