What are these changes and why do they cause the elders to fear for the younger generation and the future of the clan?
The greatest fear arises from the changes seen since the white missionaries moved into the area. The clan elders fear the loss of their cultural heritage, and are worried that the advent of Christianity will pull the younger generation from their religion and traditions. Uchendu is happy at the feast however; he says it is good for the younger generation to see a man like Okonkwo doing things in the grand old way. thus, even though Okonkwo was not as respectful as he should have been while staying with his mother's family, he represents someone who maintains the traditions of the village.
Uchendu expresses fear for the young people because they no longer know how to speak with one voice. Christianity, an abominable religion, has settled among them. He says that now a man can curse the gods and the ancestors; now a man can leave his father and brothers like a mad dog who suddenly turns on its master. The elders are fearful for the young people, and they thank Okonkwo for calling the family together. It is this breaking up (the "falling apart" of the title) that the elders most fear. And they have reason to fear it, it proves to affect their society in much the way they imagined.
Essentially, the impact of colonization is to break apart the Ibo societal structure. In some ways, this has negative effects: traditions are destroyed, and those who follow Ibo law are suddenly subject to British punishment. Many villagers die or remain in jail because of their adherence to their own rituals and traditions, rather than the recently imposed British structure. Religious conflict is brutal, and both villagers and missionaries die because of their inability to adapt to change. Okonkwo demonstrates the worst that could happen to someone who is unable to transform when history forces change. His aggressive stance toward the missionaries brings about his own destruction.