I think that there are hints that Lucas may have have the idea earlier. Vader being Luke’s father is not in any of the screenplay drafts for the same reason that even David Prowse did not know - secrecy.

But we have been forgetting another group of people who knew the secret - Del Rey. The novelization came out about the same time as the movie. According to Wookiepedia it was May 1980. Donald F. Glut, the writer, had to know about it far enough in advance to put in the correct wording (interesting to note that Luke knew Vader was telling the truth in the novel), as did the publishers, editors, proof readers, type setters, etc. But that is a group that has been good at keeping secrets for a very long time.

I seriously don’t think this was a last minute change. I think he went in with this in mind. He wrote the story and Leigh Brackett did the screenplay and on all the copies I’ve seen, that spot says “insert dialog B”. Whether he let her in on the secret or not, we can’t know, but the draft scripts that got out do not have any line at that spot. I think he had some backstory planned for Anakin that was not what Ben told Luke in ANH (what that is no one knows), but Alec Guinness made this face right before he launched into the story of what happened that just screams something more significant. That was either a lucky coincidence or a brilliant inclusion.