Biden drops out and probably need a rule change to free delegates. Easier to have Joe drop out after convention. That cuts out completely those 3900+ pledged delegates.
Party bosses pick. Wouldn’t have to be Kamala. New pick could pick his own VP too. But likely just be Kamala as pick for president.
Under current rules, if Biden withdraws his delegates are released from their obligation to vote for him. They can pick anyone. Biden can recommend, but not require a vote for someone in particular. Current rules say they should "in all good conscience" reflect the preference of the voters who elected them, but they are not legally bound to any candidate once the candidate they were pledged to withdraws.
The party will want to avoid that free-for-all, so watch for them to change the rules.