I let mine expire once and wasn't aware of it until I tried to use it. It had expired a week or two earlier. I called SWA and they let me use it without paying any fee. But that was 4 or 5 years ago, before they instituted the Freedom rewards, which in my experience has made it much harder to book a reward flight without having to convert two regular rewards to a Freedom reward. I doubt they would do that now, but you can always ask.