In the case of early voting and a ticket change, either the votes are null or they count for the new ticket. So if you are right and the courts decide that the early votes arriving prior to the ticket change are null, first we have to figure out at which exact moment ballot segregation must occur. That's going to be a wonderful court fight.
Then we have redress, in which a revote is impossible due to the "same day" constitutional requirement. That means that there is no effective redress. This is where we see judges empretzeling the law to the point where it is not recognizable. I could see a huge court battle on toss-out versus count-it-as-the-new ticket.
This could make Bush v. Gore look like a Sunday picnic. I'm not sure how it would end up after a 4-4 Supreme Court tie.
Now we are getting on the same page. So, path of least resistance is precedence of Bush v Gore, i.e., no candidate substitution, no revote.
I don’t see any legal or practical situation where the Dems win this.