I have found the easiest thing to do is call for a confirmed flight change. You can call up to 12 hours before the flight you want to change to, and if there is availability, they will put you on the flight for $50. I use this quite often, and it is really nice not having to sit around at the airport and hope I get on a standby flight. The last couple times I've flown, the standby lists have been 20-30 people deep. Just get a confirmed flight change, get your seat assignment, pay the $50 and be done.