It's now impossible for the Democrats to change their candidate. Too late losers. You can't get anyone else on the ballot.

Also let me spare you the agony of reading through all that fluff article

Strategists theorize that Democrats would have to hold a public event to symbolically transfer power to the new candidate. Biden, Obama, Clinton, Schumer, and Pelosi would publicly introduce and endorse the anointed nominee.

They would also have to convince Harris to throw her support behind the substitute, which would be a painful experience for someone so fiercely protective of their political future.