Uhhh...Demonrats!?
If they were Republicans, you would be right. But being that they're Democrats they can change whatever they want up to Nov. 5 Jan. 21.
It is really up to the individual parties. Obama cannot willy nilly pick someone according to their rules. And then you have the individual states to consider. Each state has thier own rules to when the canidates and electors have to be submitted.
I don’t remember what they did about replacing Robert Kennedy.