As it is right now their respective websites don't indicate it, but the Spark compatible model is supposed to be getting into the retail channels so at this point it will probably start showing up as old inventory clears out, but when the websites might reflect this is another issue. I can say I did order one from Amazon a few days ago to see if it was the new model and it turned out to still be the old 720 model so I sent it back. Also Samsung's website now calls the Sprint GS4 Tri-Band even if the model number listed is the old one and lists Best Buy as a location to get it (even their exclusive blue model has the Tri-Band label on Samsung's site) so it should be available there at some point, but getting one at any particular Best Buy would still probably require some luck with them having cleared out the old model stock.
