Read very closely, follow the flow of the context, and it should be readily apparent that the author first and foremost speaks of the transformation of the priesthood, (then the transformation of the Torah, which comes only by the understanding of the Testimony of the Messiah written for us in the Gospel accounts).
Hebrews 7:11-24 ASV
11 Now if there was perfection through the Levitical priesthood (for under it hath the people received the law), what further need was there that another priest should arise after the order of Melchizedek, and not be reckoned after the order of Aaron?
12 For the priesthood being changed, [transformed] there is made of necessity a change [transformation] also of the law.
13 For he of whom these things are said belongeth to another tribe, from which no man hath given attendance at the altar.
14 For it is evident that our Lord hath sprung out of Judah; as to which tribe Moses spake nothing concerning priests.
15 And what we say is yet more abundantly evident, if after the likeness of Melchizedek there ariseth another priest,
16 who hath been made, not after the law of a carnal commandment, but after the power of an endless life:
17 for it is witnessed of him, Thou art a priest for ever After the order of Melchizedek.
18 For there is a disannulling of a foregoing commandment because of its weakness and unprofitableness
19 (for the law made nothing perfect), and a bringing in thereupon of a better hope, through which we draw nigh unto God.
20 And inasmuch as it is not without the taking of an oath
21 (for they indeed have been made priests without an oath; but he with an oath by him that saith of him, The Lord sware and will not repent himself, Thou art a priest for ever);
22 by so much also hath Jesus become the surety of a better covenant.
23 And they indeed have been made priests many in number, because that by death they are hindered from continuing:
24 but he, because he abideth for ever, hath his priesthood unchangeable.
The author speaks of the disannulling of the "covenant with death" as likewise foretold in the Prophet Isaiah, and that has to do first and foremost in this context with the fact that every priest until Messiah died, and when he died, another was elected to take his place, and when that one died, another was elected to take his place: a covenant with death, which was weak because of the flesh, for all men sin and die, and thus he says that it was to be disannulled because of its weakness and unprofitableness due to the fact that all men are appointed once to die. But when you have one who is raised from the dead, and that is of course Messiah, and he has been proclaimed by the Father with an oath, "You are a priest forever, (after the order of Melki-Tzedek)", then there is likewise the necessity for a transformation in the Torah; and that comes by way of his holy Testimony which expounds the Torah in the new supernal and spiritual Way and which Testimony is written for us in the Gospel accounts, (and in the Apocalypse). This, as I said earlier, was already long expected to be the case by the Rabbins and Sages of old, and there should be no doubt that the Isaiah passage is where the author of Hebrews is drawing from in the above statements, for it concerns the "covenant with death" which Isaiah foretells will be disannulled:
Isaiah 28:9-15 KJV
9 Whom shall he teach knowledge? and whom shall he make to understand doctrine? them that are weaned from the milk, and drawn from the breasts.
10 For precept must be upon precept, precept upon precept; line upon line, line upon line; here a little, and there a little:
11 For with stammering lips and another tongue will he speak to this people.
12 To whom he said, This is the rest wherewith ye may cause the weary to rest; and this is the refreshing: yet they would not hear.
13 But the word of the LORD was unto them precept upon precept, precept upon precept; line upon line, line upon line; here a little, and there a little; that they might go, and fall backward, and be broken, and snared, and taken.
14 Wherefore hear the word of the LORD, ye scornful men, that rule this people which is in Jerusalem.
15 Because ye have said, We have made a covenant with death, and with hell are we at agreement; when the overflowing scourge shall pass through, it shall not come unto us: for we have made lies our refuge, and under falsehood have we hid ourselves:
16 Therefore thus saith the Lord GOD, Behold, I lay in Zion for a foundation a stone, a tried stone, a precious corner stone, a sure foundation: he that believeth shall not make haste.
17 Judgment also will I lay to the line, and righteousness to the plummet: and the hail shall sweep away the refuge of lies, and the waters shall overflow the hiding place.
18 And your covenant with death shall be disannulled, and your agreement with hell shall not stand; when the overflowing scourge shall pass through, then ye shall be trodden down by it.
Notice the above is addressed to the rulers of the people at Jerusalem, and that is to say, the Kohanim-Priests and Sanhedrin Elders: and that surely speaks of the inferior doctrines, teachings, Sanhedrin rulings, decrees, ordinances, dogmas, (δογμασιν, Eph2:15, Col2:14), and teachings and commandments of men and the traditions of the Elders which the Master is always railing against. The fifth chapter of Matthew expounds the same by reason of the fact that the Master is not saying "It is written" when he refutes their teachings, but rather, "You have heard it said", (and τοις αρχαιοις, which can be either "of old time" or might even be "the old timers", "the ancient ones", implying "the traditions of the Elders" handed down). That section of torah-teaching of the Spirit of Life in Messiah begins with Mat5:20,21, condemning the righteousness of the Pharisees and Scribes, which they held to be contained in "the traditions of the Elders". The Master is by his Testimony disannulling their flesh-minded covenant with death by teaching the truth about the same Torah: and in some cases they did not go far enough in their understanding of the Torah, and in other cases they had utterly misapplied what is written in the Torah by what they decreed, taught, spoke, and said, (as in Mat5:38-42).