Could the words ‘Father changes into Darth Vader’ be a reference to a Force vision, much like Luke’s Dagobah Cave experience? In the finished film Luke defeats Vader and his face turns into Luke’s face, but perhaps there was a version where Luke’s father appears to him in a dream or vision but then changes into Vader to represent the revenge that Luke feels that he needs to take.
I suggest this alternate explanation merely because if I suddenly had the idea to combine the two characters, I would simply write ‘Father is Darth Vader’, or ‘Father changed to Darth Vader’. It’s very strange to use the present form of the word ‘changes’ since it suggests something which happens within the film, rather than being a revelation of an unchanging fact.
It’s definitely possible. The full context only raises more questions.
The full context reads: “Somewhere the good father (Ben) watches over the child’s fate, ready to assert his power when critically needed. Father changes into Darth Vader, who is a passing manifestation, and will return triumphant. Luke travels to the end of the world and makes sacrifice to undo the spell put on his father. He succeeds and happiness is restored.” This is on page 7 of The Making of Star Wars: The Empire Strikes Back. Rinzler claims these words are in the notes for an “early outline” of a Star Wars sequel, but no actual dates for these notes are provided.
These notes are pretty hard to make sense of, and the fact that Ben is also referred to as “father” only adds to the confusion. But you’re right that it could easily apply to some kind of vision Luke has, or some spiritual interpretation of Luke’s journey. But the last two sentences could also be interpreted as Vader’s redemption arc. Regardless, the meaning here is a bit obscure, and it’s not certain that it really refers to the famous plot twist. And again, there are no dates on these notes, so those sentences could have been written after Brackett’s first draft.
I tend to prefer Kaminski’s hypothesis, that Vader was made into Luke’s father through the process of Lucas struggling to remove the redundancy introduced when Luke’s father (originally) appeared as another wise old Jedi Force Ghost, duplicating the role of Kenobi. If the account from Marcia Lucas mentioned by Emre1601 is correct, it’s possible that Lucas ran this problem by some of his friends, and one of them suggested the idea of merging Vader with Luke’s father.
In general, it’s very common in literature and screen-writing for a writer to merge two characters into one if one character becomes redundant. Thus, I think it’s likely that Vader was turned into Luke’s father not for the sake of some amazing twist, but rather as a solution to a writing problem caused by having redundant characters. The fact that it was also an incredible twist was of course a huge bonus.