“I don’t know the rules. Do you have a link to the rules you could post?”
No, I don’t have a link handy, but I do know that the parties are allowed to choose their nominees in any way they see fit.
Currently, both major parties conduct a primary election to determine who that person will be. You’d probably have to look up the specific rules regarding how a Democrat nominee is replaced. Try the DNC website.
Awarding the spot to anyone but the runner up in the primary election would be a huge break with American political tradition, but who knows? Perhaps the choice would be made by a hastily reconvened gathering of their delegates and superdelegates.
TAKE THE HIGH ROAD, MR. TRUMP!
Offer Hillary sincere wishes for her improved health. It not only highlights her illness, but shows you in an amazingly good light.