I think a vote for Biden counts for Biden only. If Biden drops out and Clinton replaces him, his votes don't carry over. IOW, votes are for candidates, not parties.
I always thought the best way to fuck up the dem mail-in scam was to get Biden out late, after voting has started.
Not sure, though, if someone voted Biden/Harris, and Harris replaced Biden, if her votes as VP candidate would count for her as a pres candidate, since it's not a separate ballot item. Would mean if Biden went, probably best if Harris went, too, because it could be a mess, legally.