Yes, they could opt to re-specify the payload as an upfitter/coach-builder. HOWEVER, to do this, they would assume some amount of responsibility for the change - in other words, they assume liability. They cannot, for example, just slap some stiffer springs on and add a sticker that says "new GVRW is 7500lbs". Testing, or at least design analysis, would need to be done to provide a certification of the re-rated GVRW. Put another way, they need to do enough to cover their asses if a customer has a problem and decides to sue. If they sell a truck "rerated" 1000lbs higher than OEM and the frame snaps in half after 20k miles, you can bet they'll be held accountable unless they have the receipts to show that it wasn't their fault.