I do this frequently -- in fact, usually. It's the workaround for Southwest's perceived "disadvantage" (vs. other airlines) of having to pay the walk-up fare for any close-in change of a return. I'm happy to give Southwest the, usually, ~$50 float. Yes, I am sorry to take the low fare from someone else, but I think if they'd followed the same advance purchase and other t&c as me, they were eligible for the same low fare.