Daas Yochid:
Yibum might be a good example. There was a concept of Yibum which existed before matan Torah; that’s what Yehudah marrying Tamar was about. After matan Torah, the issur of marrying a daughter-in-law doesn’t allow this type of “Yibum”.
The ??????? in ????? the beginning of the Eleventh perek discuss this.