No. There is no deadline for changing the actual candidate. There are deadlines, legal and practical, for changing the name that appears on the ballot, but the name that appears on the ballot is a mere voter convenience. The voter is not voting for the candidate, in fact. The voter is voting for an elector, and ALL of the electors (and their alternates) have been named, before the deadline.
There are practical issues with changing candidates, because absent obtaining a majority of the electoral votes, the DEM party loses the election.
But as a legal matter, Kerry can become unavailable the day before the election in November, but the DEM electors will cast ballots in December for the person the DEM party tells them to.
Thank you. Most Americans are so ignorant of the political process that they think they are voting for a specific presidential candidate. In fact, as you say, they are merely voting for an elector that is currently pledged to that candidate. And that elector can always vote for somebody else entirely.