The gate agent was more than happy to put me on the earlier flight for free, even though I was on the cheapest advanced purchase fare. I suspect its because the flight I was booked on was still showing a 40 minute delay at the time I got the gate. Flightaware shows it ended up leaving within 20 minutes of schedule, probably since the load was so light. I don't mind short delays, since WN really hustles to get the plane out, and more and more these days seems to beat the estimated time.

Ironically, I got a call from Southwest notifying me of the delay just as we were ready to push back on the earlier flight.