So back in the 70's and 80's when rodding Ford Pops and Y's etc. was the in-thing we would buy a new chassis (e.g. Jago) and then put that under an old Pop shell. Then decide the shell is rotten so buy another Pop and use the better shell (but keep first cars no. plate).

Then add running gear etc.

Shell's back then didn't have stamped VIN numbers and 3rd party chassis were not serial numbered so were we breaking the law?

Is this not about Intent? If the intent, from swapping IDs and VIN plates etc. is to defraud by claiming something is more valuable than in reality (e.g. Cosworth rebodied onto standard car shell) then, as a buyer I would not wish to pay out full wack for a car that is not genuine original. But if sold as Cosworth rebuilt using 1600L shell then no intent to deceive. Yes? I guess that sounds better than a 1600L upgrade to Cosworth spec and hence impact on resale value.