What has changed in God's intention or our understanding with respect to adultery and divorce such that the Church should change it's teaching on it?  From my perspective what has changed is the misconception of what marriage is and the reasons that people get married.  That, and a secular society that has decided to make marriage a temporary institution.  Catholics know or should know before they take those vows of the consequences of divorce; that is part of the deal with God.  

What's changed in the Church is the infiltration of liberal-minded priests who assume that people are too weak to honor their commitment to lifelong marriage.