Aquinas on Changing Laws

But, to a certain extent, the mere change of law is of itself prejudicial to the common good: because custom avails much for the observance of laws, seeing that what is done contrary to general custom, even in slight matters, is looked upon as grave. Consequently, when a law is changed, the binding power of the law is diminished, in so far as custom is abolished.  Wherefore human law should never be changed, unless, in some way or other, the common weal be compensated according to the extent of the harm done in this respect.

           St. Thomas Aquinas, Summa Theologica (Benziger Bros. edition, 1947)
           First part of the second part, question 97, article 2