Ballots are set.There’s no way you could get new approved ballots in place before the election. And for the reason you stated, it would be a very bad idea for the dems. If Biden got 30% of the vote, and candidate X, or lets say, Michelle Obama got 30%, while Trump got 40%, Trump would win with a plurality of the vote. Ballots are cast for a candidate, not a party.
Also, even if dems tried to pull off a swap or some kind of trick, state legislatures could intervene and prevent the electors from voting for a different candidate than the one who received the most votes. There was a SCOTUS case earlier this year settling this matter. ALL of the states Trump won in 2016 have R legislatures.
The dems are stuck with Joe. Now, if he wins and becomes incapacitated between Nov 3 and Jan 20, that’s a whole different ballgame.
In any event, The New York Post just tossed a huge, greasy monkey wrench into the election.