Picture this:
You find the Mopar of your dreams, a 69 Hemi Whatzit, sitting in a pasture where it was parked in 71. You ask the owner of the farm if you can take a look at the old Dodgmouth out back and he says "look all you want. I'd sell it to ya fer $235." You try to hide your enthusiasm when you discover that this car is 100 percent numbers matching and still even has the window sticker in the glovebox. You quickly make the purchase and excavate whats left of this rare car and trailer it home. This thing is a friggin mess. Every piece of sheetmetal needs replaced, the frame sagged 20 years ago, and the roof is even rusted through. You are thinking of writing this one off, but your research leads you to find that this car is the only 69 Whatzit to be equipped with both the Hemi and green bench seat interior. Rarer still is the fact that it was painted toilet seat pink from the factory, a special order color.
Then you find her rust free southern twin on Ubuy. Friggin identical except that it has the slant five and was painted nipple brown. Your clicker finger is quick, and you snipe the other bidder. Once you have that Whatzit in your garage, it occurs to you how simple it would be to do a switcheroo. Besides, the five cylinder car has no paperwork, so if you don't do the presto-chango-vin deal you are stuck with 2 parts cars. Knowing that 69 Whatzits are hard to find in your part of the country, and are always either basket cases or cost 80 gigillion bucks, it would be sinfull to scrap 2 of them instead of making one good one from the two.
Dirtbags who alter Vins and fendertags, that is another story entirely!!!!