You are in the context here of what happens after the convention, before the election. Once any given state has accepted the list of delegates for the Electoral College of each party, and printed ballots, then only that state could decide to reprint ballots or accept different delegate lists.
An arbitrary change of heart by a national party, unless it had a compelling story with it (a death, say), would be insufficient to get most states to change or reprint, and insufficient to get a majority of voters to follow. At least in most states - New Jersey being an obvious exception.
An outright loss (e.g. death) of a candidate would only put the power of selection back in the party's hands to the extent that the party could agree on a compelling alternative, quickly. In this case, compelling enough to overcome already printed ballots in some or most states with the departed candidates name on them.
As soon as it gets close to the actual election, it is simply too late to reprint ballots. Election results in the closely contested battle ground states would shift against any candidate whose name wasn't actually on the ballot.
This may work when done as a last minute surprise in a local or state election, especially if the party doing it already has massive corrupt control of the election. It fails on a national level unless the circumstances are highly favorable.
Fun speculation, perhaps not idle. I bet Bill and Hillary and many others have given this keen, critial analysis.
The twelfth amendment says that the electors will make a list including the names of the persons, and how many votes each person got. If that process is mangled, the election gets tossed to Congress.