Any number of things could have happened to them depending on how Jefferson did it.
My guess is that they would have remained with him and continued taking care of his property, but that they would get some small amount of pay for doing it.
This is what happened with many slaves freed in the past.
But then wouldn’t he be required to house, cloth and feed them? Who would pay for that? Or would his plantation devolve to something like the company towns the coal industry became (in)famous for.
My point being, once slavery was accepted, it became like holding a tiger by the tail. I believe this was the reason slave holders would free their slaves upon death. It was now someone else’s problem.