This is a great question. It might be first very helpful to give a theory of how change takes place.
Thomas Kuhn, a Harvard professor, has arguably shown better than anyone else how such a process takes place. He writes about scientific theories, but his insights can be used with ideologies. His theory is as follows:
He argues that a scientific community accepts a theory, which he calls a paradigm, over others, because one paradigm is more successful than others in solving a few problems that a group believes to be important. However, no paradigm is perfect. In fact, it is the imperfections of these paradigms (he actually calls them anomalies) that can lead to paradigm shifts.
When we use this framework with ideologies, then we can see that ideologies conflict with each other and may even bring about change. The given ideology can be called the traditional one. It is traditional as it is accepted. A new ideology is liberal, since it is new and it seeks to challenge an older one. The new ideology is able to challenge an older model because the older one is not perfect. There are anomalies.
When these ideologies come into conflict, there is a "battle" that emerges. The ideology that answer more of the questions of the people at that time will win. Let me give an example.
We can say that a heterosexual view of marriage is the traditional view. The newer or more liberal view of marriage is the belief in same sex marriage. There is a conflict between these views. The view that will most likely win will be the one that answers most effectively the questions of the prevailing worldview.