They can’t do a bait and switch. All the electors Biden received in the primaries are obligated to vote for him. He has more than enough electors to win in the first round of voting.
If he steps down before the convention, it becomes an ugly free for all. Not only that, but whoever comes out of the convention as nominee has no campaign funds, which is a severe handicap going into the general election. Biden’s funds can’t just be given to his replacement.
If Biden steps aside for the election, it’s basically an admission he is not fit to be president NOW. And that is a very serious problem for the Dems AND the country.
The best the Dems can do is hope to win the election, then deal with it.