It's highly debatable. I've asked this question about my car years ago.
Morally and ethically, my opinion is that it's an original car if it has more than 50% of its unit body structure. Legally, however...laws are generally written by people who aren't knowledgeable about cars. Most of the laws that I've read read something like (it's a rebody if the VIN tag has been moved." The problems with that are:
1) The VIN in most of our cars is mounted to the dash pad, allowing it to be removed from the vehicle without being removed from its mount
2) As a good friend of mine pointed out years ago, you could build a jig around the cowl, remove everything else and replace it, and claim that the VIN was never moved.