So why are teachers needed in the new covenant?
The laws of The new covenant are DIFFERENT from the Laws in the old. FOR EXAMPLE: no animal sacrifices or circumcision (of all males organs) required in the New covenant
If you can't be honest about the fact that the laws have been changed, then it's pointless to have a conversation.
People have a choice to cling to lies they tell themselves or pray and ask God to show them truth from delusions.
I would just argue that Circumcision was always intended to be from the heart(mind), not just the flesh.
Lev. 26:
40 If they shall confess their iniquity, and the iniquity of their fathers, with their trespass which they trespassed against me, and that also they have walked contrary unto me;
41 And
that I also have walked contrary unto them, and have brought them into the land of their enemies;
if then their uncircumcised hearts be humbled, and they then accept of the punishment of their iniquity:
42 Then will I remember my covenant with Jacob, and also my covenant with Isaac, and also my covenant with Abraham will I remember; and I will remember the land.
So by the time Malachi was written, all God's true people already knew it was the Spiritual intent of Circumcision to be from the heart, just like they knew an Israelite was one from the heart, inwardly not outwardly. That is the intent of the teaching of the Law and Prophets that was revealed by the Christ and His Apostles.
The problem is that those who were given the service of the Priesthood had rejected God's instructions and created their own version of God's Laws as it is written. When God sent His Prophets to set them straight they killed them. So God sent His Son and they killed Him as well. These preachers, the Levites, which were given the Priests office had corrupted God's Word.
Matt. 15:
7 Ye hypocrites, well did Esaias prophesy of you, saying,
8 This people draweth nigh unto me with their mouth,
and honoureth me with their lips; but their heart is far from me.
9
But in vain they do worship me,
teaching for doctrines the commandments of men.
And again;
Heb. 8:
7 For if that first
covenant had been faultless, then should no place have been sought for the second.
8 For finding fault with them,(Levite Priests) he saith, Behold, the days come, saith the Lord, when
I will make a new covenant with the
house of Israel and with the
house of Judah:
So He did as He said He would do. He became our High Priest, No more Levite Priests, He Forgives our sin, not more Levitical Priesthood sacrificial "Works of the Law" for the remission of sins.
So you are correct that there was a change in the Law, this change was the fulfillment of the change of the Levitical Priesthood given by the Word which became Flesh in Jer. 31, and the duties which were given exclusively to the Levites, have now been inherited by the Christ.
This is not taught because of the man made religious traditions we have been warned of so many times. But if you study the Word you will find this account of the New covenant to be true.