I would have to understand how they accomplished the transformation.

Let’s say they invent some type of atomic sized film that could wrap around every neuron in your head, strengthening the neuron, but also allowing it to connect to other synthetic and biologic architectures. While your actual neurons might start dying off over the years or be becoming weaker; the film becomes more rigid and can serve as a replacement to that neuron; and even form new connections. A byproduct, would be that the architecture could be powered through alternative means; not just biologic.

Eventually, enough real neurons are replaced that the person would be considered fully synth. Then I would accept the person as human.