We change a ski when there is enough innovation to make a tangible difference. Sometimes a ski is really good the way it is so we keep it until we find ways to make it better. If you change a great ski every year, you end up making changes simply for change's sake and that's not good for anyone. I don't know of any brand that makes huge changes every year. If one year people really like a ski and then the next year it is greatly changed, I think it would anger more people than anything.
Plus, this is Chris' ski. If he likes it the way it is, he won't change it until he wants something different.