Matthew Poole and continuators on Hebrews 7:12

12 For the priesthood being changed , there is made of necessity a change also of the law .

For, refers to the Expiration of the Aaronical Order, to which these Hebrews now were not bound, for that a better Priesthood and Law were to fill up their room in the Church. The Levitical Priesthood was changed and abolished to make way for this; God designing that to continue for a time, and then to expire, when the Truth perfecting it should take place.

The mutation of the Priesthood indispensably requireth the change of the Law, i. e. the legal Dispensation of the Covenant of Grace, and the bringing in with another Priesthood a better Hope, Gal. 3.17, to 27. comp. v. 18, 19 of this Chapter, even the Covenant of Grace in the Gospel-Dispensation of it. This was made necessary by the Decree of God, who determined, that both Priesthood and Law should expire together, and accordingly hath fulfilled it. For when Christ the Gospel-High-Priest had in his Person and Work perfected all of it in Heaven, he roots out that Order of Priesthood, abolisheth the Law, scatters the People which would cleave to it; demolisheth the Temple and City to which he confined the Administration, so as all Designs and Endeavours of Iews, or of apostate Christians, to repair, or to restore it, hath been ineffectual to this day.