I've cancelled and rebooked for lower fares. You just get the remainder as credit (and no cancellation fee!). I primarily pay with points, but it'll work with money too. This is one thing I LOVE about SW. For example, I am not sure what day I'll be able to fly home for christmas, so I booked two different flights in order to secure a low rate regardless of how the schedule pans out. Once I know for sure which flight I'll take, I'll just cancel the other one. All southwest requires is that you cancel the flight at least 10 or 15 mins before the flight is scheduled to depart. I know SW gets a bad rap, but I haven't had any issues flying with them. Sure, flights are late once in a while, but I've experienced that with almost every other airline I've flown.