My feeling is GL didn't decide about Anakin = Vader until TESB. In the Ep IV original novelization,
allegedly written by Lucas, when that scene comes, it goes like this:
Quote:
"How," he asked slowly, "did my father die?"
Kenobi hesitated, and Luke sensed that the old man had no wish to talk about this particular matter. Unlike Owen Lars, however, Kenobi was unable to take refuge in a comfortable lie.
"He was betrayed and murdered," Kenobi declared solemnly, "by a very young Jedi named Darth Vader."
etc.
The way I read this, Kenobi is uncomfortable (as in the film) but definitely not lying (in the screwed-up, now non-continuity of the books); he's just uncomfortable because he's telling Luke something very important, and something he also feels guilty about (in the novel he does still blame himself for Vader's fall, Anakin or not.) But there could be another reading of it, in that what he tells Luke is an
uncomfortable lie.
This is as good as place as any to mention that the original novelizations are still a continuity mess, last I heard. In my ROTJ book, you've got Vader referring to Obi-Wan as "Ben" on Endor to Luke, Ben saying Owen is his
own brother, and that Vader didn't know that his wife was pregnant at all, and more. I think in ANH, Ben also mentions Emperors, plural.
Not that any of this ruins SW or anything. Guinness's line reading does work great in the film, and I think Owen's "that's what I'm afraid of" works even better in the later context.