There is the argument that when the Party formally replaces a candidate whether that candidate withdrew voluntarily or for reasons of incapacity the Electors are free to vote for the Party's replacement. I.e., there is no "faithful elector" issue due to impossibility and, as a practical matter, who else are they going to vote for?
Which of the two possibilities sounds most reasonable?
But the electors are themselves elected. So any votes for the electors pledged to Biden prior to his replacement must be discarded. While NY,Cal, Ill, Ma would not matter, Mi, Fl, Wi, Mn, Az, Va, Pa most certainly would.