Flipping the candidate 4-5 months before the election is the dumbest fucking idea. Other than the obvious legal battles and terrible optics, the logistics of getting a unified candidate and get that information out to the public before the election is impossible.
Sen Mark Warner might as well be a Republican for attacking the Dem candidate. It won't do anything good, it will fail, and it will be used to attack Biden going forward. Being critical and shooting yourself and country in the face don't have to be the same thing.