In the Star Wars Trilogy, Luke Skywalker and his father Darth Vader go through many changes and come out with a total transformation in Episode IV. These changes are mostly for the better. In a way, they do not really change or transform, they only become who they really are.
In Episode III, A New Hope, Luke Skywalker is introduced as a whinny laborer for his aunt and uncle. He never wants to do what he is told to do. He doesn't want to stay and help his uncle, but when Obi-Wan Kenobi asks him to go with him to learn the force, he says he can't leave his uncle. As he follows his destiny, Luke Skywalker is reluctant to every calling along the way. In The Empire Strikes Back, Luke returns with a new courage about him. He is strong with the force, although he has not completed his training.
Having no fear, he approaches the lair of Jabba the Hut. This shows a strong turning point in all the films. Now that Luke has accepted who he is becoming and the power he holds, anything can happen. Through the end of this film and the beginning of The Return of the Jedi, Luke does not want to accept his fate. Finally there is another transformation and he knows he must face his father, the enemy, to bring the Empire down.
Darth Vader, Dark Lord of the Sith, was ruthless and unforgiving even to his son.
Throughout Episodes IV and V, Vader keeps up this attitude. Luke continues to believe that there is some good left in his father. He does all he can to get this out of him.
Finally at the end of The Return of the Jedi, when the Emperor is killing Luke, Vader opens his heart and saves Luke from death. By doing this he is killed and the Empire is brought down. Darth Vader's transformation is proven to be the biggest change in the trilogy and saves the galaxy.