So's speeding. Or carrying a joint or firearm across state lines for most of us. Or claiming tax deductions higher than actual. Lots of things are illegal. I'd wager you do at least one of those either intentionally or unintentionally every day.
If I sold it as the car the VIN identified, and that number matched the chassis stampings, exactly what is the impact of the change on the buyer? In this case the intent of the law is to stop misrepresentation of the vehicle that would add to the monetary value for the seller, and make the buyer a victim of fraud. The buyer has to believe the car is something more valuable than it really is. There is no misrepresentation here. Just a '73 no-pedigree car with '71 taillights, nose, and gages in a car I've said is a '73.