This is an interesting pickle the Dems have gotten themselves into. I have posted about the rules and procedures about replacing a candiidateAFTER Primaries in the recent months.
If Biden's name is on the ticket and people are voting for him right now. He gets the vote whether he stays in the race or not. ( no different than the times people wrote in Mickey Mouse ) Harris does not become the stand in allowng his votes to go to her. That can only happen on Nov 4th if he wins. They are stuck with Joe.
https://twitter.com/joshdcaplan/status/1319055755379695622