Any attempt to remove the selected candidate from the ballot in 50 states, and replace ithat candidate with another - even the Hildebeast, would not succed with equal ease or felicity in every state, and would likely fail in many or most states where it is attempted.
The result would be very confusing to the average voter, to say nothing of the potential legislative and judicial morass. Lawsuits galore would be filed and litigation scheduled and fought through. Most middle of the road voters would be immensely turned off by such an eventually and would turn against the party responsible, leading to an unprecedented debacle for the Dempocrats.
The Democrats would not permit such an eventuality and therefore would not condone Kerry's dropping out before the election. They have their horse already registered, listed, saddled up and on the track. They have to ride that horse to the end of the race, come what may.
I am speaking of political practicalities, not legalistic possibilities. Flights of fancy are not particularly interesting to me. I am willing to stick my neck out and flatly say that this nightmare scenario will never come to pass.
Now, sure, tragedy can always strike and in such cases, a way forward will always be found. I am not interested in any such cases, as they are basically without relevance to a candidate simply up and quiting because he thinks he can't win. That is unprecedented, and we are not about to see it this year.
Such has happened before at lower levels...
I certainly don't believe it will happen here, barring, as you say, a tragedy. But never say it can't happen here.
The Democrats will do whatever in hell they are told to do by the powers that have totally purchased them.
Namely George Soros (formerly employed as a teenager by the Iron Cross [Hungarian Nazi Party] in 1944. (This information can be confirmed by reading Soros' father's book--check it out on Amazon.)