This is a great question and the answer is yes and no. Let me explain.
First, when you say that something is the least changed, it is a strong statement. "Least" is absolute. In light of this, I would say that pottery is one of the least changed. This is a much safer option. There is an analogy with ancient pottery. If you look at the Greeks and Romans, their pottery, at least in method, has not changed all that much.
Second, when we consider design, then we can say that there has been change. Even a cursory glance at the variability of styles and designs show that there has been a lot of development among American Indians. Also the various Indian groups produced different styles as well. From this design perspective, the art on the pottery has changed, though it is one of the least changed.