Hello,
In answer to your question, Yes, the new barrels will fit the old build guns. You can get new barrels from the factory, but they are pricey. You would have to call for current pricing, as they have not yet added revolver parts to their new website. (It has been a long, slow, update.) Don't try emailing them, call & leave a message, they will call back.
Your best bet for a replacement barrel at a reasonable price is ebay, gunbroker.com, or auctionarms.com. The shorter barrels usually don't sell too high, as most interest seems to be in the longer barrels. I believe the barrels have been made in the following lengths: 2 1/2", 4", 6", 8", 10", 12"(rare), and 15"(very rare). You also have the option of the "light" or "heavy" (full length) underlug on the shroud, as well as "vented" or "solid" rib.
Good Luck!