The 6501 was pin compatible with the 6800, but had its own instruction set. After the lawsuit, the pins changed and we got the 6502. This meant that if you enabled 6800 support on the Apple 1 you could drop in a 6501 and use it with software made for the 6502. If you were really ambitious you could use a 6800 but then you'd have to write a new monitor ROM. There's no evidence that this was done at the time, but about 10 years ago someone did it: https://www.youtube.com/watch?v=ag6pWUhps7U

The 6800 wasn't a drop-in replacement on the Apple 1. Installing one required some hardware modifications to the board. (It's unclear if anyone who owned one ever did this.)