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.