Thats incorrect, they can tell how much weight your tag is good for based off the first few digits on the plate. Figured that out at the tag office when she gave me a 8,000 tag and I asked for a 13,000. Put the first plate back and grabbed one from a different stack.
If you raise or lower the weight on a tag you already have, then they don't change out the plate.
Also, the troopers have a online "catalog" they can search that shows the weight of just about every piece of equipment out there. They'll pull up beside you, glance at the tag, add up a rough weight of what you're towing and pull you over.
For the record I'm not on the states side, I've just been pulled and know the fines add up quick, lol.