Depending on state law, electors may be bound to vote for the specific candidate on the ballot, and their party may not have the power to direct or allow them to vote for a replacement. The Supreme Court has held faithless elector laws to be enforceable, and some call for the immediate, automatic disqualification and replacement of and invalidation of the vote of any elector who fails to vote as they are bound.
But they’re bound to do the bidding of the state legislature. Who will just slam through a law saying “vote for who the party replaced the guy with”. It’s not that tough.
In short...
In October, especially later in the month, and in early November (before November 8), the situation would have become significantly more complicated. At this point, nearly all ballot certification deadlines would have passed, many ballots were printed, and voters in some states had already cast their ballots.
This begs the question: What happens if a candidate has dropped out of the race but wins the popular vote in a state? Would the replacement nominee just receive those electoral votes? The answer lies in what that state has to say about its electors in the electoral college. The Constitution does not dictate how electors must cast their votes. But some states do. More than half the states have laws dictating how electors must vote. If the former nominee won in a state that does not have a law on how its electors vote, then, theoretically, he or she could win all of that state's electoral votes. But if the former nominee won in a state that does have a law on how its electors vote, then one would have to look at that law's fine print to see what would happen and if the state's electoral votes could go to the replacement nominee.