Back to the Future’s version of Johnny B Goode is better than the original version. And I’m saying this as someone who loves Chuck Berry.
But the phone call that follows, even if it’s a time paradox, is an unfortunate racist joke. That short scene could have been left on the cutting room floor and the movie would not have been changed at all (besides being slightly less racist).
Why is it a racist joke?
Chuck Berry couldn’t come up with the song independently; he had to steal it from a white man. I assume that’s the logic, anyway.