God has always been holy, righteous, and good, so He has always had such a conduct, and His law is holy, righteous, and good (Romans 7:12) because it is based on God's holiness, righteousness, and goodness, and because it is His instructions for how to have such a conduct. This means that the way to have such a conduct existed from the beginning before God made any covenants with man, it exists independently of any covenant, and it can not change unless God's holiness, righteousness, and goodness changes first. So unless you are arguing that God's character changed when the seed should come to whom the promise was made, then the way to act in line with God's character remained the same, as it was revealed to Moses.
Genesis 26:5 because Abraham obeyed my voice and kept my charge, my commandments, my statutes, and my laws.”
God did not leave His people without instructions prior to Moses, but rather they knew how to do what is holy, righteous, and good because they listened to God's voice. Cain and Abel had been given instructions about how to make offerings, Noah had been given instructions in regard to clean and unclean animals, and Abraham was given commandments, statutes, and laws, which refers to a way of living that clearly covered more than two specific instructions.
The law was given to teach of the love of God for the whole of His creation until the love of God would come and remedy the sins that separated God from His creation. The law wasn't given as a trap, but to make us aware of our sin and to give us a remedy from those transgressions so that we might learn and move on to lives that bring honor on to God. Paul was certainly not saying that now that now that the seed to whom the promise was made has come that we can now feel free to dishonor God by going to back to doing what He has revealed to be sin and the lawlessness that he redeemed us from.
Hebrews 7:12 is essentially saying that an eternal High Priest could not come from the Aaronic line, not that God's holiness, righteousness, or goodness or the way to live in line with that need to be changed.