Kind of a grey area. I see this question come up from time to time.
I've swapped the cab in a Toyota pickup without changing the registration or VIN. Technically maybe you're supposed to the way the law is written in a lot of states but who's gonna know? Same frame, same engine, same doors, fenders, dashboard and everything else that was original. You can easily swap the plates on the dash and doorjamb. The only place with a different VIN at that point would be the one on the firewall and who ever looks there? Plus, even with a picky inspector, if EVERYTHING else comes back to the original VIN who's gonna say anything about that? Just be honest, say the cab was ****** up so you replaced it with one from a junkyard or a private sale. Maybe keep that bill of sale with you.
People worry too much about laws sometimes, you really just gotta do what's reasonable regardless of what some book says, usually common sense will prevail. Or if you're really that worried about it, carefully cut out and weld that part of the firewall even, then it really is the exact same truck. I may or may not have also done that to various vehicles over the years...
