"For when the priesthood is changed, of necessity there takes place a change of law also," (Hebrews 7:12)
"For, on the one hand, there is a setting aside of a former commandment because of its weakness and uselessness," (Hebrews 7:18)
"Do not add to what I command you and do not subtract from it, but keep the commands of the Lord your God that I give you," (Deut. 4:2)
"For the Law made nothing perfect..." (Hebrews 7:19)
"The Law of the Lord is perfect..." (Psalm 19:7)
"The 'change' or 'transformation' that the Torah has experienced in the arrival of the Messiah primarily regards permanent atonement for sins and Yeshua serving in the office of the Melchizedekian priest," pg. 111 of Hebrews: For the Practical Messianic.Regarding the "setting aside of a former commandment" in Hebrews 7:18,
"Vs. 18-19 do not say that God's Torah was set aside; what is said is that 'a former commandment is set aside'," pg. 113, ibid.
"As far as looking to the Levitical priesthood and its animal sacrifices as being the source of perfection and reconciliation with the Creator, Believers in Yeshua should effectively consider such a system nullified," pg. 113, ibid.
"The fact that the Messiah's Melchizedian priesthood causes 'a setting aside of a former commandment' (v. 18a), of the Levitical priesthood, has to be balanced with various prophecies within the Tanach (Old Testament) which speak of the reinstatement of the Levitical priesthood and animal sacrifices in the future. In Jeremiah 33:20-22, the Lord says that His covenant with David and with the Levites cannot be broken....Ezekiel 44:10-11 describes the Levites in the Millenial Temple performing animal sacrifices...." pg. 114, ibid.
"We instead have to conclude the athetesis or setting aside of the Levitical priesthood and sacrifices (Hebrews 7:18) to either concern eternal perfection being now offered in Yeshua with such a previous system effectively nullified, and/or a temporary setting aside of this system with the priesthood once again operative at a future point for God's end-time purposes," pg. 114, ibid.
"Lane is keen to note for us, 'Its 'weakness'...inheres not in the law or its purpose, but in the people upon whom it depends for its accomplishment...Its 'uselessness'...derives from the fact that the law regulated the approach of God in a cultic sense and was able to cleanse only externally (9:9-10, 13, 23; 10:14),'" pgs 114-115, ibid.
"...Guthrie astutely concludes, 'There is no doubt that the writer [of Hebrews] does not here mean that the law itself is annulled, but that it can be discounted as a means of gaining perfection...It is characteristic of law--not merely the Mosaic law, but all law--that it has made nothing perfect. All it could do was focus on imperfection. Indeed the Mosaic law went further and demonstrated in its application that perfection was impossible,'" pg. 115.SUMMARY
So it looks like "do not add" really means "make no unauthorized changes." Because we have it on the authority of the Tanak that there is another priesthood, that of the order of Melchizedek. And a "setting aside of the former commandment" cannot be interpreted to mean that the Levitical priesthood is abolished but rather that the aspect of the former commandment as a source of perfection and reconciliation--that aspect has been set aside. And the "weakness" and "uselessness" of the "former commandment" cannot be interpreted to mean that the Law itself is weak or useless (which would contradict the Tanak) but rather that the incompleteness of the former commandment regarding the Levitical priesthood--that incompleteness is hereby nullified by Yeshua, our perfect sacrifice and mediation.