I think the crucial element making this a manner of attaining immortality is if the process gradual enough that the synthetic and natural neurons can overlap, with the electrical signals of the natural brain crossing into and back from the synthetic parts, because at that point the two are functionally identical, and the loss of the natural neuronal tissue would be like any other brain injury—something to be compensated for; and something which might change one’s identity to some extent, but not a fundamental loss or death of consciousness. Assuming it’s possible to simply continue adding more synthetic neuronal tissue, eventually the natural neuronal tissue could be a small part of the expansive whole.