You can swap any 20 pound propane tank at any place where they do those exchanges. If you do not have the label, its no problem. They take the tanks and refill them and slap a new label on. Now, it really IS cheaper getting your tank filled.
I did get a new tank a few years ago when I swapped out my round knob tank for a triangle shaped knob tank. I thought that was pretty cool. I got a new tank and the old one's they were phasing out ... just "disappeared". I couldn't have been happier.
If you have a failing tank, it's a great way to get a newer one. Otherwise, its cheaper to get them refilled at a dealership of some sort... RV dealer, ACE Hardware, some lumber yards, any propane company will refill cheaper than the exchanges.