Those with nonrefundable fares—like pretty much everyone—need to change or cancel any flights at least ten minutes before departure. If not, that’s when the sad stuff happens, as your reservation will be canceled and you’ll lose the unused portion of your ticket. Southwest announced things a few months ago, but the new rules are just going into effect now.
Airfare involved includes some of the cheaper types of tickets including Wanna Get Away fares booked after May 10. If you cancel or change your flight in advance it’s really not going to be a problem, it’s just those flights where you fail to show up at the airport and fail to call the airline.
Bottom line—just call Southwest at least ten minutes before your scheduled flight, and they’ll help you use a portion of your fare—depending on a number of factors—towards a different reservation. If not, it looks like you’ll be saying goodbye to any remaining portion of your ticket.