They’ll need to reprint all the pre-completed mail-in ballots.
Rookie. They’ll just count all the pre-marked ballots for Biden, as really being for the new candidate and the person marking the ballot really had the INTENT to vote the right way..
Unlike Senators from New Jersey, the President is not directly elected by voters, rather, he is indirectly appointed by State Legislatures who appoint the 535 members of the Electoral College (Congress granted itself 3 Electors in 1960).
Since all 50 State Legislatures, and Congress, at present use the method of citizens voting to appoint their Electors, it LOOKS like there is a thing in November called a "Presidential election", but it only LOOKS that way.
There are no "Presidential elections" in the Constitution, no "being on the ballot", no "running for President", no "nominations", no "calling states for...", and so forth.
Thus, if California votes for "Biden" when "Biden" is not "running", the California Legislature will simply appoint 53 Electors to vote for Newsom, or Whitmer, or Satan for that matter - all perfectly legal and Constitutional.
YUP
THE SUDDEN PURCHSE OF MORE PAPER/INK/PRINTING time will give them away