I was thinking about a moment from the Obi-Wan vs. Vader duel when Vader says “You should not have come back” and Obi-Wan flashes a very concerned look.
My thought was when considering a prequel rewrite, to make this bit the time when Obi-Wan realizes that it’s Anakin who is Vader, not the “other pupil.” He thought that there could be a chance (i.e., saw Anakin using the dark side more and more), but never truly realized it until this moment. Obi-Wan remains resolute despite the revelation, with his smirk towards Vader now meaning something like, “Anakin, that’s your son out there. I’m making damn sure he’s getting away from you now and I’ve got a guy who’ll help him kill you. Your obsession with me hasn’t changed since the Clone Wars, so you probably can’t sense him at this moment.”
I like the idea of Obi-Wan being in the dark about Vader’s true identity all these years, as it adds a little bit of dramatic weight to each time Obi-Wan tries to tell Luke about Vader. Heck, it even gives his final conference with Luke in ESB a bit more weight. He can’t tell Luke the truth he just found out (and probably told Yoda), so as not to give him that same disconcertion he had during the ANH duel. Obi-Wan knows that Luke won’t be able to take this revelation well, as unlike him, he hasn’t had the many years to ponder it.