The DNC can place anyone on the Democratic ticket they want.
But—they have a problem.
Early voting:
https://www.ncsl.org/research/elections-and-campaigns/early-voting-in-state-elections.aspx
As you can see, many of the states allow early voting _weeks_ before the election, and many Biden supporters will choose to use early voting.
So—if they cast a vote for Biden for President and he is no longer on the ballot, they have still cast a vote for Biden for President—and all the lawyers in the world can’t clean up that mess on aisle three.
A late replacement will mean a guaranteed Trump re-election, since he will be on all ballots and the new Democratic candidate will only be on some of them.
The “October surprise” would be that the Democrats are burnt toast.
Popular votes don’t matter. As long as you vote for the underlying electors, who have already been selected by the candidates in each state. Any vote for the candidate is a vote for that party no mater who is on the ballot or even alive. The small print on the ballot reads “for electors pledged to _____.
At least not legally.