George is the character that changes the most. He realizes at the conclusion of the story that Lennie will never change. Since Lennie's violence has escalated into murder, George must give up his dream of a farm and kill Lennie. Curley's Wife is accidentally killed by Lennie before she has the opportunity to evolve. Unfortunately, Crooks remains bitter due to the instutionalized and social racism that exists in 1930s California.
I have to say that this is a tough one because it depends on a person's point of view. I believe that George was the one that changed the most. George was always responsible for Lennie, but he was not one to commit an illegal act. He has had to make a very hard decision out of the love for Lennie. George has always had the capacity to change but with the responsibility for caring for Lennie serving as a weight, he had no chance to change. I believe that with Lennie gone George will become a more independent and stronger person. He will have to come to terms with his actions, but he has the clear head and ability to move forward in a positive manner.