Objection 1. It would seem that the natural law can be changed. Because on Sirach 17:9, "He gave them instructions, and the law of life," the gloss says: "He wished the law of the letter to be written, in order to correct the law of nature." But that which is corrected is changed. Therefore the natural law can be changed.
Objection 2. Further, the slaying of the innocent, adultery, and theft are against the natural law. But we find these things changed by God: as when God commanded Abraham to slay his innocent son (Genesis 22:2); and when he ordered the Jews to borrow and purloin the vessels of the Egyptians (Exodus 12:35); and when He commanded Osee to take to himself "a wife of fornications" (Hosea 1:2). Therefore the natural law can be changed.
Objection 3. Further, Isidore says (Etym. 5:4) that "the possession of all things in common, and universal freedom, are matters of natural law." But these things are seen to be changed by human laws. Therefore it seems that the natural law is subject to change.
On the contrary, It is said in the Decretals (Dist. v): "The natural law dates from the creation of the rational creature. It does not vary according to time, but remains unchangeable."
I answer that, A change in the natural law may be understood in two ways. First, by way of addition. In this sense nothing hinders the natural law from being changed: since many things for the benefit of human life have been added over and above the natural law, both by the Divine law and by human laws.
Secondly, a change in the natural law may be understood by way of subtraction, so that what previously was according to the natural law, ceases to be so. In this sense, the natural law is altogether unchangeable in its first principles: but in its secondary principles, which, as we have said (4), are certain detailed proximate conclusions drawn from the first principles, the natural law is not changed so that what it prescribes be not right in most cases. But it may be changed in some particular cases of rare occurrence, through some special causes hindering the observance of such precepts, as stated above (Article 4).
Reply to Objection 1. The written law is said to be given for the correction of the natural law, either because it supplies what was wanting to the natural law; or because the natural law was perverted in the hearts of some men, as to certain matters, so that they esteemed those things good which are naturally evil; which perversion stood in need of correction.
Reply to Objection 2. All men alike, both guilty and innocent, die the death of nature: which death of nature is inflicted by the power of God on account of original sin, according to 1 Samuel 2:6: "The Lord killeth and maketh alive." Consequently, by the command of God, death can be inflicted on any man, guilty or innocent, without any injustice whatever. In like manner adultery is intercourse with another's wife; who is allotted to him by the law emanating from God. Consequently intercourse with any woman, by the command of God, is neither adultery nor fornication. The same applies to theft, which is the taking of another's property. For whatever is taken by the command of God, to Whom all things belong, is not taken against the will of its owner, whereas it is in this that theft consists. Nor is it only in human things, that whatever is commanded by God is right; but also in natural things, whatever is done by God, is, in some way, natural, as stated in the I, 105, 6, ad 1.
Reply to Objection 3. A thing is said to belong to the natural law in two ways. First, because nature inclines thereto: e.g. that one should not do harm to another. Secondly, because nature did not bring in the contrary: thus we might say that for man to be naked is of the natural law, because nature did not give him clothes, but art invented them. In this sense, "the possession of all things in common and universal freedom" are said to be of the natural law, because, to wit, the distinction of possessions and slavery were not brought in by nature, but devised by human reason for the benefit of human life. Accordingly the law of nature was not changed in this respect, except by addition.
Source: Summa Theologica (New Advent)