GTP - Happy New Year back at ya!
I think your thread brings out that there are at least 2 ways to look at the question: one legal/registration & the other in acceptable restoration practice.
1. - For the first - each state would have their own interpretation & regulations on the matter, and one would need to check with their particular state. There is probably some level of commonality state to state.
1.5 - A sub-set of of that may be how your insurance company interprets the respective state law and their coverage policies & terms.
For both the above & #2 below, this would apply in the 90+% parts change-out or body-swap scenarios, but the state DMVs/BMVs concerns are that folks aren't ripped-off by a dishonest dealer, shop or home mechanic with a franken-car wreck that has been cut & 2 welded together (as opposed to a true resto.).
2. - For the second - in most classic car restoration circles I think it's generally been accepted that extensive restorations - whether by individual parts or by an entire body shell, can be considered a restoration of the original car - so long as the original s/n or VIN is transferred to the completed resto, and there's not an orphaned "shell" running around.
With body-on-frame cars this is more straightforward as the frame & drivetrain remain, whereas the thought on it has had to morph a bit with unibody cars since the frame is integrated into the body shell, and they have been around since the 1940s-50s (VW Bugs, 356s, 911s/912s & domestics & foreigns), and their restorations with body shells appears to be just as accepted today - as noted by a couple of prior posters.
Finally - in both cases 1 & 2 - re-numbering VIN/SN of an existing shell/body of another car gets into muddy waters & is more sticky with newer cars, because of the registration of the donor shell/body as another "legal vehicle" - particularly when it serves to change the character of the vehicle, including when it's key parts or just the VIN/SN stampings transfered into another car body which has been registered (e.g.: changing a 914/4 conversion with a legal original 914-6 VIN or body parts).
Even more muddy is when clips are used from another car which includes then the old & "new" VINs/SNs in a Franken-car (914/4 to 914-6 is again a good example).
So maybe 3. is these latter muddy cases, as there are many Brass Age cars from the late 1800's through 1920's, & some later cars, which have needed to be reconstructed from various cars parts.
In any case, I believe the key is to be honest & straightforward about the steps taken to repair & restore a vehicle in both #1 & #2 (& #3).
In my case, I explored the possibility of using a body-in-white as a better resolution of rust & accident damage issues which pushed the price to comparable or well past to what a new body shell would cost. I wouldn't have fudged on the fact, and would've gone through the appropriate steps to register it & have the "resto story" told for any CW events, etc.
But I really want the car to be more reliable & rust-free for me to enjoy for the rest of my life (I'm 59), then to pass it on to my son to do what he pleases. I'm not at all interested in a "hobby career" of dong 914 nor other restos, but just in enjoying my finished car which has a lot of history.
And...yes for those thinking he's crazy - I am, but I'm also open to jumping ship on my resto (becomes a parts car), if I could find an excellent 73 914-2.0 or maybe an original 914-6 in an OE color I like.
Hope that clears up the two aspects of this excellent subject you've brought up!
I think it is one which will eek along with new info added from time to time, so I'd encourage you to bump it every once in awhile to ask others to add in what they find out as they go through restos of all types.
Time to go watch some Rugby - oops! .....I mean "R-word"!
Cheers!
Tom
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