Katallo, I've only done it once, and I called SWA directly to do it. The rep couldn't give me the same price quoted on the airline (which I totally don't understand lol), so she just walked me through the process. It was quick, painless, and I got the same flight. You could ask them to check seat availability before you cancel/rebook. Any time I've called SWA to ask a question (and I've called A LOT), they've been very helpful.