I don't think Luffy is supposed to change per say. I think he is written more like a folk hero who is supposed to inspire others (meaning both reader and characters).
As I read the story I noticed that the world of OP becomes more complex, but Luffy's core character is supposed to be someone who isn't negatively influenced by external forces. He's always going to bounce back.
Is this the type of answer that you're looking for
@bennbeckman