Political parties that wish to substitute a new presidential or vice-presidential candidate are free to do that, thanks to the existence of the Electoral College, even if it is too late to reprint ballots.
In all states, the election laws say that presidential and vice-presidential candidates’ names appear on the November ballot, not as candidates, but as markers for competing slates of presidential electors. In most states, but not all, the ballot says “Vote for Presidential Electors”. And then the list of presidential and vice-presidential candidates follows.
In 1968, George Wallace had a stand-in vice-presidential nominee, Marvin Griffin (former Governor of Georgia). On October 5, Wallace chose his actual running mate, Curtis LeMay. Only five states printed LeMay’s name on the ballot; 43 states printed Griffin’s name; and there were three states that didn’t print vice-presidential candidates on the ballot for any ticket (Alabama, Arizona, and North Dakota). But Wallace told the nation that his presidential elector candidates, if elected, would vote for LeMay. Wallace carried five states and all of Wallace’s electors did vote for LeMay in the electoral college. Of those five states, Mississippi, Arkansas, and Georgia November ballots had listed Griffin, but it didn’t matter; LeMay received the electoral votes.
In 1912, the Republican nominee for Vice-President, James Sherman (who was the incumbent vice-president) died before the November election. His name was on the November ballots, but in December, the Republican presidential electors voted for Nicholas Murray Butler, the new vice-presidential nominee for the Republican Party. The party’s national committee had met late in October and chosen him.
This article has the Democratic Party’s rules for replacing a presidential or vice-presidential nominee.