That is irrelevant.
If she "replaces" Kerry, she does not need to replace him on the ballot.
All that is needed is for Kerry (or his widow, I don't think HRC has made a final decision on THAT) and the DNC to instruct electors chosen on the Democratic line to vote for HRC instead of Kerry.
This is perfectly legal AND (for the first time in HRC's life) Constitutional.
It's an uphill chore to educate. Sometimes I get energetic, and explain "the way the electoral college works," and why the name on the ballot is merely a convenience for the voter and for the party.
It's nice to see posts that at least correct the facts.
As for the efficacy of a substitution, I don't know. I can see Kerry resigning due to scandal -- that would be accepted by the public. But I don't see a way for the DEM party to pull a majority of the electoral votes in November, no matter who the nominal candidates (Pres/VP) are.
Interesting scenario, but even under those circumstances her chances of winning would be remote at best. How many independent voters -- especially those who generally aren't terribly interested in politics -- are going to vote for a candidate whose name isn't even on the ballot?