You are right—man cannot reverse what GOD has done, BUT GOD CAN—and did . Hebrews 7 says GOD (not man) changed the law; and not just about Priest, He changed all of the law from Judaism to The law of Christ. Even in Matthew 5-7 Jesus was changing the law. The old law taught “you shall not murder”, but Jesus said, “But I SAY, you can’t even hate your brother…”. The OLD law said, “Do not commit adultery” but JESUS SAYS, “Do not even lust for a woman..” and (new law) if you lust, you have committed adultery in your heart. That wasn’t Old Testament teaching, that is the new law of Christ. The old Jewish law permitted divorce for various reason, but Christ changed the law, and under the new law of Christ there is only one reason why a person can divorce their spouse and that is for sexual immorality. The old law said “you shall not swear falsely, but I SAY do not swear at all…” The old Jewish law said, “An eye for an eye” but JESUS SAYS do not resist…be meek, be giving. The old Jewish law said, “ Love your neighbor and hate your enemy”, but I SAY love your enemies. That’s just the beginning. Colossians tells us that feast days, new moons, dietary laws concerning meat and drink, SABBATH days, all no longer are bound on us—we are not required to keep those any more, so don’t let anyone judge you for not keeping those old laws nor let them tell you you must keep them. Col. 2:14-16. Circumcision is another old law that has been done away. Galations 5:6 “In Christ-meaning in the new law of Christ—neither circumcision nor uncircumcision avails anything.” He says, “ You observe days, and months and seasons and years, I am afraid of you lest I have labored for you in VAIN.””
So in Hebrews 7 when he said THE LAW HAD TO BE CHANGED, what was changed was much more than just the laws concerning priests. The Jewish laws about murder, adultery, swearing, an eye for an eye, divorce, vengeance, and hating your enemy; feast days, sabbath days, new moons, festivals, dietary laws, days, months, seasons and years and —a major one—circumcision, just to name a few! The whole Jewish law was changed. Hebrews says it was OBSOLETE AND VANISHING AWAY. Colossians says Jesus TOOK IT OUT OF THE WAY AND NAILED IT TO HIS CROSS.
Jesus said in Matthew 5 that it would only last UNTIL He had fulfilled it. (Verse 18) There were many prophecies about Him in the Old Testament and all of those had to be fulfilled before His new law came into effect. We know Isaiah 53 is a messianic chapter about the death of Jesus and the events surrounding His death. That must be fulfilled before the old law vanished away. Once He died on the cross all things were fulfilled about Him. Jesus said with His dying words, “It is finished.” This harmonizes with Hebrews 9:16-17 that says before someone’s “will” can come into effect, that person must be dead. We live on this side of the cross. The old Jewish law has been “taken away” and we are now living under the New Law of Jesus Christ. Ignatius, a second century writer was right. “ Judaism prohibits “grace.”