It depends on if your original ticket was refundable or non-refundable. Technically, SW will refund the difference as a credit to your credit card if your original ticket was refundable. If the original ticket was non-refundable, then SW gives you an air travel credit for the difference that they hold for up to 1 year and you may use to purchase another ticket in the future. There have been times, however, when you call SW and the person will credit your credit card even on a refundable fare, but they certainly do not have to do this. Most tickets purchased are non-refundable fares because those are the least expensive.