Quite a few changes happened to Unuofia during the 7 years of exile. Perhaps if Okonkwo had been present in his village during that time, the changes would not seem so drastic, but they are glaring changes after being gone for 7 years.
One change is the dominant religion practiced. It is no longer the traditional Igbo religion. Christianity has replaced it. In fact, even Okonkwo's own son, Nwoye, has converted to Christianity. That embarrasses Okonkwo deeply.
Coinciding with the religious change is a racial change. There are white people living with the Igbo people. White missionaries brought the religion and continue to live in the village.
Another change is in the government and ruling court system. As with the religion, a white man's court has replaced Igbo traditions. For example, Okonkwo was exiled for 7 years, but Aneto was hanged by the white man's court for a similar crime.