I understand using creative editing and reshot footage of the Vader costume to imply that Anakin and Vader are different characters. The only thing I'm clueless about is how it will address Ben's later comment of "Was a pupil of mine until he turned to evil". I mean come ANH, the audience will know something's up because this phantom figure was never said to be Obi's apprentice previously...and the only one that was his apprentice was the one that he supposedly murdered.

Maybe you have this covered, I don't know. I hope so :)