#1 - Vader goes "Kenobi killed your father" (what everyone, including the actors, thought, up until they saw the premiere of Empire. Only Lucas, James Earl Jones, and like 3 sound editors knew the truth. Hmm, that sounds familair....)
#2 - Luke still does the header, and the rest of ESB goes 'as is'.
#3 - Everything is ROTJ is fine right up until Dagobah. Now we have an arguement between Kenobi and Skywalker. Revealing either
a)- Anakin's death was an accident. it was Vader vs Kenobi + Anakin, and Anakin got killed by a accidental swing from Kenobi
b)- Anakin had turned to the Dark Side before Kenobi defeated him, and Vader is Anakin's replacement. Vader betrayed Anakin to become the Sith apprentice.
#4 - During the Death Star duel, Luke is still cool as a cucumber, but has none of the family ties. Prior, he's not trying to turn Vader, he's looking for information about Anakin. During the duel, instead of trying to appeal to Vader's former ID, he's appealing to his former life as Anakin's friend.
#5 - Duel finale either
a) - Goes as is, with Vader redeeming himself to save his best friends son.
b) - Luke kills Vader, and ends up fighting the Emperor.
There are other options, but that's getting into 'Alterate Universe rewrite of Return of the Jedi' territory.
Luke only searched out Vader and went to the Death Star willingly to help turn his father.
Otherwise, he'd have stayed on Endor and helped destroy the Death Star that way, figuring Vader would die in the blast.
If we take Grand Admiral Thrawn's "Palpatine was using the Force to coordinate his troops" theory at face value, without killing Palpatine, the Empire curbstomps the Rebel fleet.
Otherwise, ignoring that, the question is whether Vader sensed Luke going down to Endor. If he did, then Vader goes to Endor, seeks out Luke, and they duel to the death. If he didn't, Luke and the strike team successfully destroy the shield generator, Vader dies when the Executor
is destroyed, and the Emperor is killed when the Death Star explodes.