It is.
But, as a seller, destroying the tag before selling the pad at least leaves the seller in the clear.
Anything beyond that as a buyer, check your state laws. Some states allow changing it if the change is "witnessed" by a LEO. In some case the LEO will actually switch them for you.
Also, if the same VIN is going back on the same car, most LEOs will not care. All it takes is ONE that does to cause problems for you though. So, again, check your state laws BEFORE moving a tag.