For a long time I have told people that it takes very little to change a Class 2b truck's capabilities to a Class 3 but I would only do this if I already had the truck and planned to keep it or it there were some restrictions where I live that limit my use with a Class 3 registered truck.
The GVWR sticker is a warranty placard plus it allows the manufacturer to build/option to different Class standards (for example TPMS and commercial operator license requirements). Most States do not look at manufacturer GVWR and will ask the owner what GVWR or GCWR they wish register under.
Even though I now drive a Class 5, I don't make it a blanket statement that everyone else also should. I chose this size due to my payload requirements which most people towing would never utilize.