They are probably trying to arrange that now, but the ballots are printed and a vote for Biden cannot reasonably be counted for someone else. Harris can't become president unless many millions of people write her name in (such a thing actually happened in Connecticut, where Joe Lieberman lost a primary to Ned Lamont, but then won the general election for his long-time Senate seat as a write-in!) or else Biden wins and is inaugurated, then steps or is pushed aside.