It seems to me that a disease that inflicts gross damage to substantial volumes of brain pretty much destroys the relevant information, in which case there probably isn’t much more line from “mentally declined grandfather” to “miraculously restored grandfather” than from “mentally declined grandfather” to “grandfather miraculously restored to someone else’s state of normal mental functioning” (complete with wrong memories, different personality, etc.).
It seems to me that a disease that inflicts gross damage to substantial volumes of brain pretty much destroys the relevant information, in which case there probably isn’t much more line from “mentally declined grandfather” to “miraculously restored grandfather” than from “mentally declined grandfather” to “grandfather miraculously restored to someone else’s state of normal mental functioning” (complete with wrong memories, different personality, etc.).