Who says that they're going to a hub-and-spoke model? That would hardly make sense. They've acquired a bunch of gates at ATL, which is great for Southwest. That doesn't mean that they're going to "hub" there in the traditional sense.
I believe AirTran uses a hub and spoke model. It would be very difficult to unwind this from an operational standpoint. ATL is a key airport, but it's quite congested and thus may delay Southwest's turnaround of aircraft.
