This one actually changed in my head as I was planning to write it down, to be honest (he says as he continues his tale of how to rework the scripts of the six Star Wars films into a more consistent epic.) Originally, it was going to be a case of, "Don't change a thing," like Episode Four, but there's something that's nagged me about this movie ever since I saw the prequels, and that's Yoda. Because the whole point of the series is that Yoda is wrong. His philosophy is ultimately sterile, with no room for love or redemption. He's not the Wise Old Mentor, he's stern and unbending and ultimately, Luke proves him wrong by redeeming Anakin with the same love that Yoda insists is selfish and destructive. And that doesn't come across as well as it could, because Yoda's so awesome and well-done as the Wise Old Mentor that we just naturally want to believe him. (Look at all the Expanded Universe material. Everything from the RPG to the post-Star Wars novels uses Yoda's portrayal of a Manichean struggle between the Light and Dark Sides, not Luke's ability to love and fear and hate and grieve. Luke is balanced, that's what the prophecy that Lucas never really explained very well is all about.)
So the change that brings all that into sharp relief? When Vader lands his ship on Hoth, Luke is still there, moving from his snowspeeder to his X-Wing. The hangar is blocked off by an ice fall, and while he's using the Force to clear it, he tells Luke, "Obi-Wan never told you what happened to your father, did he?"
Changes the film a bit, doesn't it? Suddenly, Luke isn't just a passive student accepting Yoda's philosphy, he's wondering why his father can't be saved. He isn't just blindly refusing to listen when Yoda tells him to abandon his friends, he's telling Yoda that the empty, sterile philosophy of placing a higher value on causes than on people is as wrong as Vader's desire for total control. His fight with Vader takes on a whole new dimension, as he tries to find a spark of good in his father while defending himself and instead ends up injured, weakened, clinging to the edge of the abyss in Bespin as Vader tells him that his only options are to give into his rage and fight, join Vader as his new apprentice, or be destroyed. And instead of simply falling, Luke's last gesture takes on a whole different meaning as he chooses to fall, rather than fight his father.
It does ruin the cliffhanger, of course, but by this point, anyone who's watched the films 1-6 knows that Vader is Luke's father anyway. (Just witness the torturous reworking of the scene between Vader and the Emperor in the DVD edition of the film, where Vader and the Emperor both try to avoid saying what they both know: "Oh, by the way, Luke's your kid!") Might as well let that bit go and try for the extra tension in the Dagobah scenes.