Since it has never happened before, no one knows how such a scenario would play out. If Biden drops out, the DNC would be able to name a replacement, it would not necessarily default to Harris who gathered how many delegates before she dropped out of the Presidential race?
What to do with the votes already cast? Any votes cast for Biden aren’t really for Biden, but rather Electors pledged to vote for Biden. Since Elector candidates are chosen by their parties for past party loyalty, I have no doubt that they would simply change their votes to the DNC’s choice. Would this cause chaos and lawsuits—no doubt.
As for ballots already printed, I suppose dems could write in their new candidate’s name and have them counted. Knowing dems, write-ins for the new candidate and votes for Biden would be counted as votes for the new candidate.
Interesting question would be, who would the dems pick to take Biden’s place?
If Biden drops out, that’s a lot of New Democratic ballets you forge.