What are some examples of a war or conflict which produced a positive change in society?
It is said that history is written by the victors, and as such, wars are generally cast as having brought about positive change, though whether the changes were worth the price in lost and ruined lives is another question. But some conflicts seem to have objectively led to positive moral change.
World War II, for example, not only led to regime change in reactionary, despotic countries, it brought about a transformed view of Judaism in the western world, for even the victors had to search their souls to examine their own complicity with the holocaust. Why did they not receive more of the Jewish victims of persecution? Why had they not worked harder to establish an Israeli state? Why had they not eradicated their own anti-semitism? This kind of moral examination led to greater humanity, at least for some.
Earlier, the horrendous bloodbath of the Thirty Years War, fought over religious differences that after it was all over did not seem all that important, led to a weariness with unreasoned religious conflict that opened the way for the Enlightenment. In the same century, the English Civil War paved the way for a limited monarchy, with an elected parliament in control of the affairs of state. In the eighteenth century, the American Revolution allowed for a successful experiment in a republican form of government based on democratic principles.
War is an undesirable way to bring about positive change, but nevertheless can do so.
There are any number of wars and conflicts (depending on exactly what you mean by “conflict”) that have produced what most people would call positive changes in one society or another. Often, this has happened when an objectionable government or institution has been overthrown by the war.
One clear example of this was the American Civil War. While the war was horrible, it did lead to the end of slavery. This is surely a positive change.
As another example, there is World War II. This conflict was the worst in the history of the world. However, it at least managed to change the governments and societies of Germany and Japan in particular. It got rid of the Nazi regime and it turned Japan away from militarism. Both societies are now free and democratic. This would likely not have happened without the war.
Conflicts can also produce good results. Perhaps the most obvious example of this is the Civil Rights Movement in the United States. That could clearly be called a conflict within the society. There was much political conflict and even some violence. But the movement resulted in a much more just and equal America.
So, while war and conflict are not ideal, they can lead to good results at times.