Here's a new craigslist Gem

[daveSanborn]

what is the difference in buying a new body and putting a VIN tag in it from a wreck you have sitting under the old oak in the back forty and taking that old wreck in the back forty and cutting 90% of the car off and replacing it with new metal?  The later has been done by many and some right here on this board.

I'm with ya Mel, it doesn't make much sense, but legally you answered your own question.....

Legally you can replace ALL of the metal/whatever on a car.... but you can't move a VIN from one vehicle to another.  If the VIN tag needs to be removed during any part of the repair, most states require that a DMV official bare witness to the event.