Objection 1. It would seem that human law should be changed, whenever something better occurs. Because human laws are devised by human reason, like other arts. But in the other arts, the tenets of former times give place to others, if something better occurs. Therefore the same should apply to human laws.
Objection 2. Further, by taking note of the past we can provide for the future. Now unless human laws had been changed when it was found possible to improve them, considerable inconvenience would have ensued; because the laws of old were crude in many points. Therefore it seems that laws should be changed, whenever anything better occurs to be enacted.
Objection 3. Further, human laws are enacted about single acts of man. But we cannot acquire perfect knowledge in singular matters, except by experience, which "requires time," as stated in Ethic. ii. Therefore it seems that as time goes on it is possible for something better to occur for legislation.
On the contrary, It is stated in the Decretals (Dist. xii, 5): "It is absurd, and a detestable shame, that we should suffer those traditions to be changed which we have received from the fathers of old."
I answer that, As stated above (Article 1), human law is rightly changed, in so far as such change is conducive to the common weal. 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. Such compensation may arise either from some very great and every evident benefit conferred by the new enactment; or from the extreme urgency of the case, due to the fact that either the existing law is clearly unjust, or its observance extremely harmful. Wherefore the jurist says [Pandect. Justin. lib. i, ff., tit. 4, De Constit. Princip.] that "in establishing new laws, there should be evidence of the benefit to be derived, before departing from a law which has long been considered just."
Reply to Objection 1. Rules of art derive their force from reason alone: and therefore whenever something better occurs, the rule followed hitherto should be changed. But "laws derive very great force from custom," as the Philosopher states (Polit. ii, 5): consequently they should not be quickly changed.
Reply to Objection 2. This argument proves that laws ought to be changed: not in view of any improvement, but for the sake of a great benefit or in a case of great urgency, as stated above. This answer applies also to the Third Objection.
Source: Summa Theologica (New Advent)