Yes, just upgrade the springs. Or just add Sumos. No reason to over-think this. You've correctly identified the component driving the lower GVWR. The sticker only reflects the configuration as it left the factory, and is intended to inform YOU how it was configured. Nobody is going to weigh you, your insurance *probably* doesn't exclude changing springs (but check! Cheaper online policies exclude coverage if there are ANY suspension modifications), and the only thing illegal is if you tried to mis-represent the original GVWR at the time of resale.