This is just a wild question that's been speculated on a little bit in the past.

What if Emmett Brown was the teenager while Marty was the "old geezer"? How would it make a difference if it was in 1938 (in actuality, Doc was 17), 1985 (with the ages of Marty and Doc flipped around), or 2033 (in actuality, Marty would be 65)?

Boy, it's hard to even began to imagine what 2033 would be like in the BTTF Universe. Although, I did say that there was a hippie/psychedelic revival by 2045 - so maybe technology would start to step back a little, as that generation might gain a new appreciation for nature. One can hope, anyway.

If Marty and Doc switched roles, it would be a totally different movie.

Yeah, that'd be about the size of it. It's interesting to speculate over. Doc would probably act old for his age, while Marty would act young for his age. It'd be a strange movie.