Don't know when they changed the model year, but back in the '70's, it was common practice with Mopars, particularly in Pro Stock, to keep up with the current model year. The 1970 and 1971 Motown Missile Challenger is the same car. The Mopar Missile Duster started out as a '72 and it was also updated to newer years. The '72 Cuda was only run as a '72 before being sold to Mike Fons.
Unfortunately, I don't know of any specific examples of model year changes to any Stock or SS cars, but I know it was done, usually for weight-to-HP advantages with different engines as well. Still done today.