Let's look at things philosophically,
What part of the car if changed, affect the OWNERSHIP/LEGAL STATUS of such vehicle.
Assume I have a truck, I get into an accident and the frame is bent, I pick up another frame at a junk yard, private party ect and swap it in. obviously the frame numbers no longer match the vin, is it or should it be illegal? after all the frame was purchased legally, it wasn't stolen. Same argument can be made for the engine , subframes or any other part that can be changed....that includes the cabin.
For example: on some trucks there were no numbers stamped anywhere but the frame AND the vin tag/s that were riveted to the dash, door frame ect
does changing the frame effects legality? maybe in some other countries but in no state in the US that I'm aware of.
how about changing the cab and riveting that vin plates on it? dito!
Now back to the case in point: as far as law enforcement is concerned the car's identity comes from the vin tags, the numbers stamped on the cab/frame interest no one, unless they are reported stolen, that will get you in hot water...otherwise...no body cares.
Back in the late 90's I moved to Reno from Socal, at the time I had this 79 Toyota pickup 4x4, the cab mounts were all gone(the floor was all rusted out) so it was much easier to swap in another cab, I transferred the dash and vin tags off the old (white) cab to the new (blue) cab, among with everything else such as doors seats ect.
In Nevada a vehicle that comes out of state and is to be registered is required to get a "vin check" you take your vehicle into the dmv and an inspector verifies that your vin numbers matches the title ect.
When I took it in the inspector commended me on a job well done!, I guess my rivering skills exceeded his expectations... did he care about or ran the vin on the firewall? hell no, imagine a complete white truck with a blue cab....it was obvious what was done here...
Some will make the argument of "vin fraud" as a crutch....but lets examine the definition of "fraud": "wrongful or criminal deception intended to result in financial or personal gain"
As long as one does not lie about the act (swapping a body in our case) there is no fraud. if one has vin tags/firewall for 00015 and transfers it all to a say 10589 (just for an example), then tries to pass it as a 000015 (to increase its value)...now we have a fraud case. its that simple.