It’s s different with trump because he has already secured the nomination. Biden was just the presumptive nominee, which is why they have no standing because there is no nominee yet.
After that, there is probably a period of time where the wheels are in motion to put people on the ballot where they cannot turn back.
Like do you think the day before the election they could just swap out a candidate? What about printing ballots and distributing them? There has to be some kind of cut off.