[Publius]

Up until the day that the Electoral College votes (December 19), the national committee of the party can in fact change the ticket.

[JediJones]

There are various state laws that stop electors from voting for a candidate who was not the election winner or that invalidate their votes. It would take a lot more research to see just what the electors are allowed to do in each state if the president-elect dies or bows out before they vote.

There's no federal law that mandates how electors must cast their votes; theoretically, if the candidate to whom they were pledged dies and their party has not made a preferred successor clear, electors can vote for their party's VP candidate, a third-party candidate, or a leading preconvention contender within their own party. Under this scenario, however, individual state laws have the potential to make things murky, given that each state has the power to determine exactly how its electoral votes are to be cast and distributed.