Anything's possible, but I really doubt that this will happen. I don't doubt that SWA is exploring such an option, but they explore LOTS of things that are worth investigating without ultimately being worth doing. I doubt that this will be the one off the wall idea that gets accepted & makes the other 19 that get rejected worth investigating, for the following reasons:
1. The -700 is the "wave of the future" at SWA. Why expand the -300 fleet?
2. "If they're cheap enough..." it might be worth doing, but with USAir's need for $, I doubt that they're making any eye-watering cheap offers for their jets. Maybe, but I doubt it.
3. How much hardware conversion would be required? Swap out first class seats, galleys, bulkheads, install HUDs, remove ACARS printers & overwater gear, etc. Not a show-stopper, but it's all additional costs. Oh, along with repainting the outside.
4. I'm sure the jets would all be in
pristine 
maintenance condition by the time SWA gets them.
5. SWA is not all that far from starting to retire some of the oldest -300's; how much life could the USAir airplanes have left on them?
None of this meant to suggest that the rumor is wrong, or that such a deal wouldn't be worth looking into. For the right price, all of the above could be managed & it could be an excellent deal for SWA, and maybe the used aircraft market is so oversaturated right now that a price that low really could be negotiated. But I tend to doubt it.
Of course, I didn't see Philly coming, so I could be wrong!