One of the great ways to generate controversial biblicals is to use the concept of changing the priesthood. Priesthood represents the group or individual that is between God and men. It would be someone who takes problems to God and God generates solutions, or sends warnings.
The first priesthood we find mentioned is the Melchizedek priesthood in Genesis 14:18 which says that the King of Salem was the priest of the Most High God. Here we have the first priesthood mentioned in the Bible.
The managed question is when was the pact made with that priesthood, ie when was Melchizedek told to be a priest? In Hebrews 7, 1 to 4 we see that Melchsedek has neither father nor mother, neither beginning of days nor end of days. Thus such a being is defined without beginning and without end. Said he remains a priest forever. So we have that Melchizedek remains a priest forever. A being who has no father, mother, beginning of days or end of life, and who is a priest forever, that being would not have a pact, he would be the original priest. A priest who would manage the law for men, but not the laws of the commandments contained in ordinances. It is the law that should be inscribed in the hearts of men.
In Hebrews 7: 5 it defines that the Levites received the tithe, according to the law given to the people of Israel. But reading Hebrews 7: 6 to 8 defines that Melchizedek, who received tithes, is in a higher priesthood than Levites, so that he defines that the Levites are mortal and that Melchizedek remains alive. Note that the Melchizedek priesthood is long lasting, while the Levitical priesthood is deadly.
In Hebrews 7:11 demonstrates the imperfection of the Levitical priesthood, to the detriment of the priesthood of the order of Melchizedek, and in this form Christ who was not counted in the order of Aaron, who is the lineage of the Levites, Christ is defined by the order of Melchizedek, the eternal priesthood.
And now we have the key verse in Hebrews 7:12 that says "For when the priesthood is changed, there must also be a change of law". And now we must ask some questions, what would be the law that changed?
Melchizedek was a priest before the Levites, and the text says that the Melchizedek priesthood is forever. Which defines that the Melchizedek priesthood never ended. For if you are a priest forever, then you don't finish. But the Levite priesthood, which is said to be inferior to that of Melchizedek, is ended. So changing the law is relative to the old laws.
What defines that the laws of the commandments contained in ordinances, these were annulled, as they comprise the organization of a specific people. But the order of Melchizedek represents the priesthood of all men. And so Melchizedek's order was never limited to a group of people. Thus the laws defined in the Melchizedek priesthood are the same as those given to Adam for the organization of all the people of the world. And not for a specific group.
The laws of the Melchizedek priesthood are linked to the law of love, and which is represented by the laws inscribed on the tablet of the heart.
Reading Titus 2:11 we have that only the grace of God manifests itself as the savior of all men, which clearly defines that at no time in man's history does salvation occur by human merit. Thus the Levitical priesthood never saved anyone by works. No one was ever saved by works. What defines that the royal priesthood is that of the order of Melchizedek, and not that of the Levites.
Thus the law that changes is the law that separates a people. For the true law, which has always existed, and which will always exist, and is related to the laws inscribed on the table of the heart of man.
If we read Galatians 3:28 we see that there is no separation between human beings, all are descendants of Adam, and are children of God. And so Christ, who is of the order of Melchizedek, is a priest for all humanity. Thus we have that Melchizedek, a being without a father without a mother without genealogy, an eternal priest, is a priest of all men. Since men are saved before Abraham exists, and before Moses wrote the law of the commandments contained in ordinances. Just as ordinances represent a people's civil system, the laws that remain are the laws for all men.
For example the law of not killing, is that an ordinance law? Since Adam, killing is wrong, it exists before the Levites, it exists in the Levites and it exists after the Levites. So when it says that changing the priesthood changes the law, that law is defining the law of the priesthood. If it were the whole law, then killing that was forbidden would become allowed, and that is not true. You can't kill yourself. This is a non-ordinance law of commandments. Other laws are to honor father and mother, not to covet, not to adulterate, and all laws that have the same foundation. These laws do not change because they are not linked to the priesthood, they are linked to what is universally correct.
What changed therefore was the rules of the priesthood, which defined a people. We can see this idea in the text in the first Corinthians 7:19 that says that circumcision is nothing, but the important thing is to obey God's commandments. See that circumcision is a law that defines a people, that law has been changed, but God's commandments remain. God's commandments were not changed because of the change in the priesthood, the law of the priesthood was changed.
If God's commandments had been changed, then right would be wrong, and wrong would be right, and evil would be right. What is totally meaningless, so the commandments of God that are not contained in ordinances, it remains. So that the Melchizedek priesthood, defined for all men, continues, and now with Christ the Priest according to the order of Melchizedek.