I am not a lawyer, but what is clear is that each State sets its own laws. By the constitution, States are in charge of elections.
What I have heard is that Biden has to release his delegates, who are already bound to him. Many states have already had their primaries completed with the Biden/Harris ticket winning.
Sending those electors to the Convention and letting them choose someone else is going to be a grey area.
If they choose Harris, it’s pretty sound. When a president steps down, the VP becomes president, so there is definitely precedent and a legal basis.
But if Biden releases his delegates and lets them vote for anyone? That will be challenged and it will go to the supreme court. And SCOTUS is corrupt enough to find some flimsy legal excuse that helps Republicans.
So yeah, that’s what I’ve heard. But I am not an expert.