Objection 1. It would seem that human law should not be changed in any way at all. Because human law is derived from the natural law, as stated above (Question 95, Article 2). But the natural law endures unchangeably. Therefore human law should also remain without any change.
Objection 2. Further, as the Philosopher says (Ethic. v, 5), a measure should be absolutely stable. But human law is the measure of human acts, as stated above (90, A1,2). Therefore it should remain without change.
Objection 3. Further, it is of the essence of law to be just and right, as stated above (Question 95, Article 2). But that which is right once is right always. Therefore that which is law once, should be always law.
On the contrary, Augustine says (De Lib. Arb. i, 6): "A temporal law, however just, may be justly changed in course of time."
I answer that, As stated above (Question 91, Article 3), human law is a dictate of reason, whereby human acts are directed. Thus there may be two causes for the just change of human law: one on the part of reason; the other on the part of man whose acts are regulated by law. The cause on the part of reason is that it seems natural to human reason to advance gradually from the imperfect to the perfect. Hence, in speculative sciences, we see that the teaching of the early philosophers was imperfect, and that it was afterwards perfected by those who succeeded them. So also in practical matters: for those who first endeavored to discover something useful for the human community, not being able by themselves to take everything into consideration, set up certain institutions which were deficient in many ways; and these were changed by subsequent lawgivers who made institutions that might prove less frequently deficient in respect of the common weal.
On the part of man, whose acts are regulated by law, the law can be rightly changed on account of the changed condition of man, to whom different things are expedient according to the difference of his condition. An example is proposed by Augustine (De Lib. Arb. i, 6): "If the people have a sense of moderation and responsibility, and are most careful guardians of the common weal, it is right to enact a law allowing such a people to choose their own magistrates for the government of the commonwealth. But if, as time goes on, the same people become so corrupt as to sell their votes, and entrust the government to scoundrels and criminals; then the right of appointing their public officials is rightly forfeit to such a people, and the choice devolves to a few good men."
Reply to Objection 1. The natural law is a participation of the eternal law, as stated above (Question 91, Article 2), and therefore endures without change, owing to the unchangeableness and perfection of the Divine Reason, the Author of nature. But the reason of man is changeable and imperfect: wherefore his law is subject to change. Moreover the natural law contains certain universal precepts, which are everlasting: whereas human law contains certain particular precepts, according to various emergencies.
Reply to Objection 2. A measure should be as enduring as possible. But nothing can be absolutely unchangeable in things that are subject to change. And therefore human law cannot be altogether unchangeable.
Reply to Objection 3. In corporal things, right is predicated absolutely: and therefore, as far as itself is concerned, always remains right. But right is predicated of law with reference to the common weal, to which one and the same thing is not always adapted, as stated above: wherefore rectitude of this kind is subject to change.
Source: Summa Theologica (New Advent)