We have had a few situations where SWA has frequent flights out of our airport to the same place, but there is a serious weather delay. In order not to stack up departures on a day where everyone is going to be behind, they sometimes cancel earlier flights that they know will not be able to leave on time, then re-book the passengers on the later flights. The worst delay I've seen in this situation was 5 hrs. Actually, I like it when they do this, because then you don't have to hang around the airport waiting to hear when the flight will actually be able to depart; you can leave and go somewhere else for awhile, even home if it is close enough. (They will keep your checked bags in this situation; you don't have to re-check them.)
I've never had them cancel my flight or change the departure time of record long ahead of the date. They do sometimes change the flight numbers, but that's not really material; the flight still usually leaves at the same time, it just is labeled with a new number for whatever reason.
