To my knowledge this pretty much never happens without some huge, lengthy disaster be it manmade or natural or otherwise sustained pressure from an external society. For the latter case, the Ainu of Hokkaido are a good one. In 1200, the Ainu grew several different grains and agriculture was an important part of their diet, although fishing and hunting were also quite important. A few groups also raised pigs and made pottery. They lived in villages and did not make many fortifications and carried on some trade with Japanese merchants.
300 years later, the Ainu only tended small gardens at most since hunting, fishing, gathering, and imported Japanese grains dominated the diet. Pigs had long since stopped being raised and they relied on imported Japanese pottery. Their villages often had large hill forts, since the Ainu competed violently over the rights to trade for all manner of things with the Japanese.
Not true at all. Even in the general area there were still large mound complexes for a century or two later, and they were only abandoned because the people who made them left. The decline of Cahokia led to an influx of population and Cahokia-esque styles and architecture to the southeast in Kentucky, Tennessee, and Georgia who continued living in a similar manner until the 16th century when drought, epidemic disease, and increased warfare stopped the construction of large mounds and formed new ethnic groups because the populations became very mixed.
The only real social change after that point became the emergence of a far more egalitarian society compared to the one before where chiefly lineages received certain preogatives and privileges. Their religion and rituals were more or less the same as ever since they still maintained eternal flames and sacred spaces for their deceased.
They were not centralised urban states. There is very little evidence Cahokia made vassals out of even its nearest neighbours, even if Cahokia's rulers were respected for the ritual complex they commanded. Cahokia was a ritual center for a confederation that would have essentially been the same as the large confederacies encountered by De Soto, which in turn were simply larger-scale, more socially stratified versions of their direct descendents like the Creek or Cherokee.