Originally Posted by Triloge
First Citizen would disappear if Marty fixed everything, considering Doc did when Emmet didn't see 'Frankenstein.' Apparently you don't even have to prevent your own existence to fade out, it can happen if you sufficiently alter the timeline to the point that you're essentially a different person. I just realized that provides an alternate explanation for Future Biff fading out in the Part II deleted scene.
(Incidentally, I like the fact that Marty actually says 'fade out,' which he previously only did in a deleted scene from Part 1.)
EDIT: A team member explained that in another thread. Doc's gone because time travel was never invented, so he never went to the future to get all those surgeries, thus his lifespan will now be shorter in this timeline than in the correct one, meaning he should be dead at his age, so he fades from existence.
Yeah you're probably referencing the thread I created thinking docs disappearance was a goof until it was explained. But anyhow according to the BTTF rules of time travel, if a time traveller changes the time line, they still only remember the old time lines. Marty is shocked at the end of part 1 to see the 'new mcfly's, neither doc or marty remember the alternate 1985 in part 2, doc doesn't seem to remember dressing marty up in the silly getup or seeing his own tombstomb at the beginning of part 3. So first citizen brown should probably not remember any of the time travel adventures and possibly even marty visiting him in 1955 and would retain all memories of the first citizen timeline if he were with marty in 1931 when marty restores that timeline.
Look at it this way; in part II doc explains that restoring the timeline transforms it around everyone including Jennifer and einstein (who were not with them when they went to 1955 to fix it) and seems only Doc and Marty remember the rich biff timeline. But suppose for arguments sake, when doc knocks biff out, they put him in the delorean and bring him to 1955 when they fix the timeline; biff should still have his memories from the alternate timeline when he is rich (he'd be the only one remembering that entire timeline) and not remember the current one where he is an auto detailer in 1985. Same concept with first citizen brown hence why marty will have to go back to the 1930s alone.