If Biden drop out of the race now it will be for severe health reasons, and he will most likely also have to resign as president and it might very well happen this weekend with all the "Batman signals" going out from DNC now. He have advanced Parkinsons and Alzheimers - that mean exponentially degeneration of brain and muscle function.

Im not sure Harris will be the DNC nomine, that is a different story. But if they want to get all the Biden campaign funds (they will need those), then they cant swap Harris, cause its impossible to move those funds to lets say a Newson/Whitmer campaign or similar.