Of course, “America” was a huge place before 1500 AD. It still is not made up of one country today and it was made up of many more small political entities before 1500 AD. Therefore, it is not as if we can say that there was one transformation that occurred in all or most of America during that time.
To the extent that there were “major governmental changes” they took place in only a few locales. In these few places, smaller, less formal systems of government gave way to centralized formal governments. These centralized governments formed the civilizations that we remember like the Incas and the Aztecs.
Before these centralized governments formed, people in the Americas were organized along the lines of bands or tribes. These were relatively egalitarian groups that were not very large in population and which did not have very formal or hierarchical systems of government. Gradually, societies in some areas got bigger and came to have much more centralized governments. They needed such governments to control the larger populations that could not be controlled through the less formal systems of band or tribal government. Therefore, more centralized governments were formed.
This is one major change that took place in the governments of some American societies before 1500 AD.