Can a human become a celestial?
Humans and other corporeal beings are essentially different from celestial beings. A human can never become an angel or demon (or vice-versa). There are several mentions of humans “becoming” demons in some rulebooks. These references are to the process where a human soul is stripped of Forces and then are grafted onto a demon (including some memories). To outward appearances, they may look the same, but the human is dead.