How do you think that is going to happen? There are already approx. 20 states past the ballot deadline making a replacement at this point improbable. Push it month, to late September, and the number doubles making it an impossibility. Keep in mind the Dems won’t be trying to push around a mamby pamby RINO, but Trump. He would sue the everliving bejeesus out of anyone trying to extend the ballot deadline.
On November 8, you won't be voting for the president, but for presidential electors pledged to a presidential (and vice presidential) candidate. The relationship between a presidential elector and his party's national committee is a private one.
You could well cast your vote for electors pledged to the Clinton/Kaine ticket (bite my tongue!), and then the electors vote in December for the Biden/Warren ticket because Hillary fell over and died. In 1872, the Democratic presidential candidate did just that right after the November election, and the Democratic National Committee communicated with their electors via registered mail to tell them that the party's ticket had changed.