I am aware of the role of the Electoral College.
They couldn't change the candidate after the deadline so would have to vote for electors associated with the former candidate.
It would be worth a lot just to watch the confusion such an event would engender amongst the RAT voters.
I don't think it would cause any confusion at all. See Widow Carnahan (who was not on the ballot). They will know exactly who to vote for, and the DEM get-out-the-vote machine is second to none. I think the DEM voters would pull a DEM lever, even if the "name" recited on the ballot was "to be determined."