DNC doesn't set election law. States do...by the authority given them by congress via the election clause in the constitution.
In Some States the time to set your ticket has passed. Certain things have to happen for that window to be opened up. 25th ammendment, Impeachment or death of the President in office. Resignation doesn't get it.
I'm a Crazy Scots-Irishman Blue Blood... It's probably not a good idea to listen to me...every thing I say is of my opinion. I'm not a good place to get investment advise...