I may be incorrect, but to run for president you need to fill out all the proper paperwork before a certain date (that's already passed). If the DNC plans to swap out candidates, they would have to replace Biden with somebody who has already filled that paperwork, such as Warren or Buttigieg. Like I said, I may be incorrect on this, but even with the DNC's backing, it would be impossible for a candidate to get on the ballot in the majority of states if they swapped out Biden for someone like Hillary or Michelle. They would be entirely reliant on write-ins. Their best bet would be to swap for Sanders but the DNC hates him so they'll never do it.

EDIT: Here's a list of every candidate. https://ballotpedia.org/List_of_registered_2020_presidential_candidates

I may be incorrect, but to run for president you need to fill out all the proper paperwork before a certain date (that's already passed). If the DNC plans to swap out candidates, they would have to replace Biden with somebody who has already filled that paperwork, such as Warren or Buttigieg. Like I said, I may be incorrect on this, but even with the DNC's backing, it would be impossible for a candidate to get on the ballot in the majority of states if they swapped out Biden for someone like Hillary or Michelle. They would be entirely reliant on write-ins. Their best bet would be to swap for Sanders but the DNC hates him so they'll never do it.