Let's throw this question around for a while.
How could we write episode III so that Obi-Wan knows that Anakin Skywalker becomes Darth Vader, the audience doesn't, and the surprise that he is will be preserved for TESB? Without showing that Anakin is Darth, how would we explain Obi-Wan/Ben's explanation of what happened to Luke's father in Ben's home? We would know that what he tells Luke isn't true, and not just from a certain point of view
. Unless there is another person pretending to be Darth Vader who pretends to kill Anakin. But then how would we explain that Obi-Wan clearly knows it is him at the end of episode IV? We know that Ben never trained "Darth Vader."Remember what we are trying to figure out is how to preserve the surprise of TESB, keep Episode IV unaltered, and make Episode III satisfy those requirements.
Hit me.