I have registered and titled my car in three states, now (Massachusetts, then Rhode Island and now South Carolina) as a white, 1969 VW Convertible.
Massachusetts was first, when I first got the car on the road, and I told the nice lady at the DMV (she really was nice, too) that the original, red, sedan body rusted out (which it had, all I saved was the central tunnel) and I replaced it with a convertible body. Her only comment was; "Oh! My brother has a body shop and he does that sort of thing all the time!" OK......cool....filled out all the forms she gave me, answered all of the questions honestly, and they (the Commonwealth of Massachusetts) re-titled it from the previous owner as a White, 1969 VW convertible. Their only beef was that I didn't re-title it and pay sales tax when I originally bought the Sedan seven years previously, so they assesed me a $25 tax penalty (and I only paid $300 for the original car!!)
Moved to Rhode Island, where the local police dept. has to personally inspect the VIN of any car older than 20 years prior to registering it. The VW VIN is still there on the central tunnel (I'm no fool), nobody found anything wrong, so it was re-titled (again) in R.I. as a White, 1969 VW Convertible.
Moved to South Carolina ("How Y'All Doin?") and they accepted everything from Rhode Island (as did State Farm Insurance) and it's still registered and insured as that same, White, 1969 VW Convertible. I pay a slight premium on insurance to get a bit better coverage than an "original" '69 VW convertible, but that's not much, either.
Seems to work for me.....but I also understand that many other cars are not in the same boat (IM's and Becks, with their tube frames and new VIN's come to mind there).