I LOVE Southwest. If the fare changes you can just go into their website and change your flights to show the new fare. It will show you how much of a credit you'll get. The credit is there for your use for one year from the time of the original purchase. And what's nice is that you can use that credit for anyone.
As I said, I only flew them once. I wasn't impressed. They weren't bad once you got in the air, but every segment was overbooked and it seemed like it took forever to get volunteers to take later flights, get the planes boarded and finally get in flight.