A month or so before the election, Biden will probably have an “issue” and step aside for Big Mike or Gruesom.
At this point, the only way to replace Biden as Democrat nominee is for him to resign from the Presidency, in which case Harris would be sworn in as POTUS, along with a VP.
As the incumbent, it would be fairly routine for her and her VP to inherit Biden’s role as Democrat nominee.
That way, whoever controls the Democrat party could force Harris to pick Michele Obama or Gavin Newsom as VP and later force Harris to resign.