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.)