I'm not too sure which character changed. Who changed the most and how over the course of the novel?
The book is so ladened with the power of redemption on so many fronts that you can make a case for nearly every major character as having undergone change and evolution. I am going to go with the sucker's bet here because of who I am and side with Salinger. I guess I am more sentimental to this choice with his passing, but I think that Kinsella showed him change to the greatest extent. The fact that he has stopped writing upon his meeting with Ray is a powerful element. His withdrawal from the world reflects his own dissatisfaction with it and the use of the only weapon he has. Yet, through both baseball and the natural idealism that Ray possesses, Salinger begins to emerge from his shell, from his withdrawal, and into an active agent of the world again. We can see that his promise to write again once he goes into "the other world," reflects that he has become empowered enough to give back what the world needs. I think that this represents a powerful change.