At this point he cannot be replaced on the ballots. At least in most states. What they can do is kind of nominate replacement (probably Harris for president and ?? for vice president) and run campaign theme “vote for Biden is actually vote for Harris”. That has ample precedent - There had been dead people running and sometimes winning. But it would really be a mess. Especially since he is still alive - all the media, campaigns, public relations, operators, Chinese trolls etc. would had to admit that there were pushing a zombie!
I think their pla is to replace Joe with someone besides Harris.
She’s NOT liked, by the base.
She’s a throw down running mate...pun (not) intended.