Timing is everything. The big question, strategically, is how long the Dems wait to make the switch.
If they go too early, it's a fast version of Trump vs Harris. They already knew she couldn't beat him straight up which is why she dropped out around Iowa. If they gave it more than 2-3 weeks before the election, Trump would immediately tweet that there is plenty of time for multiple debates and that's he's available to do so. We shrink the general election down to 3 nights in 14-21 days, and we still see plenty of unlikable Harris.
But if they go shorter - the whole world sees how deceptive they are and they lose a lot of trust. Trump goes on the offensive that it's yet another move to steal the election - forcing an election without the people getting to vet the candidate.
It's a crapshoot how this whole thing would be interpreted by the American people, but it's clearly a strategy the Dems are keeping alive