This article from The Hill is dated 5/2/2020, but it could have been written yesterday:
Next, they worry that another shoe could drop regarding questions related to Biden’s son Hunter and his business dealings. Lastly, some Democrats are concerned about Biden’s age and possible cognitive issues — a concern that some also have expressed about Trump.
For those reasons, and a few more, might we envision Biden being privately talked into retiring from the race or voluntarily doing so himself? Let’s assume that happens just before or just after the party’s convention, in whatever form it is held. What would happen then?
The process is actually fairly straightforward. If Biden quits the race right before the convention, delegates would select a new nominee. If he drops out right after the convention, members of the Democratic National Committee would pick their replacement candidate.
...one particular, truly out-of-the-box combination stops the discussion in its tracks: Hillary Clinton as the nominee and Barack Obama as her running mate.
I'm sure the delegates will give all Dem candidates other than Hillary a fair shot just like the DNC did to Bernie Sanders.
Big Mike - Hillary 2024 incoming