What would you call the effect when an oppressed group is finally acknowledged and given power, but then the group starts using this newly acquired power to oppress others or gets corrupted?
It's been called the "cycle of oppression":
The oppressed group behavior model suggests that persons who are in groups that are subordinate to more powerful groups in their society learn certain behavior patterns that, although necessary for their survival, lead to a cycle of further oppression
I don't know if the term has gained much traction (with this meaning). But it is based on the philosophy of Paulo Freire, who argues it in a bit more detail, as summarized in IEP:
Freire warns the oppressed against becoming oppressors on two counts: (1) whether the oppressed gain power and use this power to oppress their previous oppressor; or (2) in the case of the oppressed gaining power over other oppressed people and becoming their oppressors, as they seek their own individual liberation. The danger of a previously oppressed person becoming an oppressor is due to their ambiguous duality. Freire points out that the oppressed are at one and the same time both themselves (the oppressed) and the oppressor, whose consciousness they have internalized. Due to this ambiguous duality and the internalization of their oppressors, the oppressed seek to become like the oppressors and share in their way of life.
A more general concept is that history repeats itself, perhaps in "disguised" ways.
Some ancient Greeks posited a more elaborate version... the Kyklos:
According to Polybius, who has the most fully developed version of the cycle, it rotates through the three basic forms of government, democracy, aristocracy, and monarchy and the three degenerate forms of each of these governments ochlocracy, oligarchy, and tyranny. Originally society is in ochlocracy but the strongest figure emerges and sets up a monarchy. The monarch's descendants, who because of their family's power lack virtue, become despots and the monarchy degenerates into a tyranny. Because of the excesses of the ruler the tyranny is overthrown by the leading citizens of the state who set up an aristocracy. They too quickly forget about virtue and the state becomes an oligarchy. These oligarchs are overthrown by the people who set up a democracy. Democracy soon becomes corrupt and degenerates into ochlocracy, beginning the cycle anew.