Ballot rules for a party change near an election vary from state to state. Simplest thing would be to Simply state that Hillary Clinton delegates will be voting for (Biden, Kaine, Sanders, etc.) rather than HRC.
In Missouri, when Mel Carnahan died just before his re-election, they put his wife in as a stand-in, and she won.
But the ballots weren’t changed, voters still technically selected Mel Carnahan.
Ballots couldn’t be changed. Missouri Constitution strictly forbade such.