Some aspects are covered already for the nominations themselves (e.g. What is the procedure for choosing a new Republican candidate for the US presidency if the nominee withdraws or is removed? and A presidential candidate drops out after nomination). These focus one the respective national convention's nomination.
This question is about having a name on the ballot.
At least this year (2020), the deadlines for ballot access have passed in July (See example of Kanye West's announcement considered too late: https://www.independent.co.uk/news/world/americas/us-election/kanye-west-2020-running-for-president-legal-possible-forbes-interview-deadline-trump-a9608596.html), but the conventions are only taking place now (in mid August).
Question: What would happen to the ballots if either of the parties would not nominate the presumptive candidates, or they would not accept, or the nomination would swap the presumptive president and vice-president role?