Vader was always there, it was just a matter of him being changed from the original actor to Hayden Christensen because he was more recognizable.

But in context, it doesn’t seem to make much sense. Why does Darth Vader age down, while Obi Wan is still Alec Guinness instead of Ewan McGregor? If it’s supposed to be reflective of the last time he was “good”, why isn’t it him at the moment he decided to save his son and kill the Emperor?