God changes the Law at times.
At first, God tells all the Israelites they will be His priests. (Ex.19:5-6) Soon after, the people make a golden calf and only the tribe of Levi agrees to stand with Moses for the Lord.
Through Moses, God set up a priesthood to operate exclusively under the tribe of Levi and it was done in conjunction with the Law.
After having established Levites as priests perpetually, fast forward to Jesus as explained in Hebrews 7. Now via an oath, God suddenly breaks through this perpetuality and starts a new & final priesthood with Jesus being the High Priest forever. He changes the Law.
“Now if perfection was through the Levitical priesthood (for on the basis of it the people received the Law), what further need was there for another priest to arise according to the order of Melchizedek, and not be designated according to the order of Aaron? For when the priesthood is changed, of necessity there takes place a change of law also.”
No longer is this according to the Law, where only Levites can be priests. This is according to some external-to-the-Law oath. God is clearly above the Law, not equal to it. And as indicated in the text below, AFTER the Law, as in post-Law.
“For the Law appoints men as high priests who are weak, but the word of the oath, which came after the Law, appoints a Son, made perfect forever.”