With a new priest there must be a change in the law the book of Hebrews tells us. Therefore I believe that everything Jesus wants us to know about the Old Testament He brought it over to the New.
Yes there was a change in the Law but what Law?
Heb 7:12 For the priesthood being changed, of necessity there is also a change of the law.
Let's read the previous verse...
Heb 7:11 Therefore, if perfection were through the Levitical priesthood (for under it the people received the law), what further need was there that another priest should rise according to the order of Melchizedek, and not be called according to the order of Aaron?
We are speaking of the change from the Aaronic (Levitical) Priesthood BACK to the Melchizedek Priesthood. Notice here...
Gen 14:18 Then Melchizedek king of Salem brought out bread and wine; he was the priest of God Most High.
The Priesthood was originally the Melchizedek Priesthood. God gave the Aaronic Priesthood to Aarons sons but Christ was re-installed in the Melchizedek Priesthood at His resurrection.
Now, what Law was changed? Hebrews 7 tells us...
Heb 7:14 For it is evident that our Lord arose from Judah, of which tribe Moses spoke nothing concerning priesthood.
Christ was a Jew of the tribe of Judah. He could not serve in the Temple or the Aaronic Priesthood...
Exo 28:1 "Now take Aaron your brother, and his sons with him, from among the children of Israel, that he may minister to Me as priest, Aaron and Aaron's sons: Nadab, Abihu, Eleazar, and Ithamar.
Only Aaron's sons could serve as Priests. Christ gave this Law and since He will not BREAK the very least of His Laws, it had to be changed before He could become Priest again...
Luk 16:17 And it is easier for heaven and earth to pass away than for one tittle of the law to fail.