According to the explanation given by our Lord this does not mean that a truth revealed during the Old Covenant is no longer true; nor that the fundamental principles of righteousness are changed. It means this:
The Old Covenant had numerous ceremonies and contained many institutions, all of which were types of Christ, intended to teach the children of God the Plan of Salvation. By the sacrifices God demonstrated the awful consequences of sin; the holiness of God was seen in the separation of His sanctuary from the things of the world; the necessity of repentance, the Atonement, and the duties of the Priesthood, etc., were all typified. But the life and Person of Christ were the fulfillment of all such types and typical institutions. Consequently, we now look to Him for the precepts and examples which before His advent were foreshadowed in the services under the Mosaic Law. In that sense the Law of Moses was set aside for His more perfect demonstrations. It is the mode, not the subject, of instruction that has been done away with.
The Apostle Paul says: "But before faith," that is, the Gospel of Jesus, "come, we were kept under the Law, shut up unto the faith which should afterwards be revealed. Wherefore the Law was our schoolmaster to bring us to Christ, that we might be justified by faith. But after that faith [the Gospel] is come, we are no longer under a schoolmaster. For we are all the children of God by faith in Jesus Christ." (Galatians 3:23-26)
Continuing, Jesus said, "Behold I say unto you, that the Law is fulfilled that was given unto Moses. Behold, I am He that gave the Law, and I am He Who covenanted with My people, Israel: therefore, the Law in Me is fulfilled, for I have come to fulfill the Law; therefore it hath an end. Behold, I do not destroy the prophets, for as many as have not been fulfilled in Me, verily I say unto you, shall all be fulfilled. And because I said unto you, that old things hath passed away, I do not destroy that which hath been spoken concerning things which are to come. For behold, the covenant which I have made with My people is not all fulfilled; but the law which was given unto Moses hath an end in Me. Behold, I am the Law, and the Light; look unto Me, and endure to the end, and ye shall live, for unto him that endureth to the end, will I give Eternal Life. Behold, I have given unto you the commandments, therefore keep My commandments. And this is the Law and the prophets, for they truly testified of Me."
How simple, yet how grand, how plain and how comprehensive are these teachings, both in regard to Himself and to the Law which He had given to the forefathers of the Jews and to the Nephites! If men would receive these instructions in the plainness in which they were given, how much controversy would have an end, how much dissension would never have had an existence!