Here's the method they can use to replace Biden after the convention and pick whoever they want to replace him on the ticket.  This bylaw was updated in 2022.  But really, it wasn't necessary.  The DNC has the power to rewrite their rules anytime.  What they do need is political cover.  That's why they have to delay any such switcheroo until it's far too late to run a primary.  
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I'm not predicting Biden will step down.  I can see where it is a personal benefit to him to stay in office as long as possible.  I think he just has to keep from croaking or becoming incapacitated -- in which case Kamala takes over and the DNC isn't likely to want the minority rebellion if they replace her on the ballot (again, unless they swap in Michelle, etc.)

END OF MESSAGE

The Dem National Convention to designate their candidate is Aug 2024.
The Election is Nov 2024.
Christmas is Dec 2024.

http://www.youtube.com/watch?v=s_sssHJUSmY
It's astounding
Time is fleeting
Madness takes its toll
But listen closely
(Not for very much longer)
I've got to keep control
I remember doing the Time Warp
Drinking those moments when
The blackness would hit me
(And the void would be calling)
Let's do the Time Warp again.