In Hebrews, Auctor makes the following observation after his quotation of Jeremiah 31 concerning the new covenant.
When he speaks of a new covenant, he has made old the first. Now what is growing old and wearing out is already fading away. (Heb 8:13)
What Preterists think this means is that Auctor is arguing that Jeremiah predicted a future time when the new covenant was going to come and the old covenant would grow old and be soon to pass away. The common interpretation of the passage is to understand it as Auctor arguing that the old covenant is currently in the process of passing away in his own day, but that it has not quite occurred yet. Hence, Preterists see this as a reference to the coming of Christ in A.D. 70, since that is the big even about to happen and when the old covenant is said to have passed away completely.
First, I want to point out again that in Jeremiah 31, the difference between the old and new covenant is where they are written and how they are communicated. Jeremiah is addressing the Jewish community that is going to be without a temple, sacrifices, and any of the external means through which God had communicated the law to them.
Hence, the text is arguing that God will write His law upon their minds and hearts so that they will be directly taught of God, rather than having a need to have physical demonstrations and illustrations of holiness through ritual or an external law written on tablets of stone.
Instead, the new covenant will see the law written upon the very beings of the exiles themselves. “But I will make a new covenant with the whole nation of Israel after I plant them back in the land,” (31:33). In fact, the whole context relates God's restoration of Israel from the exile, the rebuilding of the temple, and the establishment of the priesthood. But the renewed covenant is needed, one that differs from the first in terms of where it is displayed, because none of the external pictures exist after they are wiped out by the Babylonians.
Fast-forward to the Auctor's time. Auctor is making the argument that the Old Testament teaches the necessity of Christ. Christ, after all, is the capstone of the new covenant, and indeed, even its foundation. He argues then that the reason why Christ's blood of the new covenant is necessary is because the old covenant is no longer in effect. It has already passed away. The external religion that existed in rituals and sacrifices were shadows of the covenant to come in Christ. Hence, once Christ had come, the shadows became ineffectual and unnecessary. This is what Auctor picks up on in Jeremiah.
His argument is that because Jeremiah used the word "new" Jeremiah was implying that the external covenant written on tablets and existing in rituals was "old." Auctor then points out that what is old is worn and is already passing away.
Where many people have misunderstood what is being said, it seems rather clear that Auctor is saying this of Jeremiah's day, not his own. In other words,Jeremiah, not Auctor, used the word "new." Jeremiah, not Auctor, implied by his use of the word "new" that the external covenant was old. Jeremiah, not Auctor, was implying that what is old is worn out and already fading away. In Jeremiah's day. In the 6th Century B.C. In fact, that's the context of Jeremiah. The old is already passing away and the new is already coming, so that the exiles can worship God even without the external religion, since now it will be written on their minds and hearts.
So Auctor is not arguing that the old covenant is currently passing away in his own day, as though it did not take its last breath at the coming of Christ and His finishing work on the cross; but rather that if it was already fading away in Jeremiah's day, then it was certainly finished off by Christ in His death, resurrection, and especially ascension.
Hence, his entire argument in Hebrews is that Christ has already fulfilled the sacrificial laws, He is already fulfilling the priesthood by being at the right hand of God the Father, He has a better tabernacle in the presence of God the Father to do His priestly work, etc. There is simply no need for the old external covenant when one has the new better one now. This is his argument to Jews leaving Christianity. It is not that they will have something better in terms of the covenant, but that they currently do through Christ's fulfilling the rituals seen in the external covenant law via His death, ascension, priesthood, tabernacle, etc.
The text of Hebrews 8:13 is simply being misread. If I say that Napoleon said that he wore blue to the Battle of Waterloo, and I say, "by blue, he implies that he did not wear red, and what is not red is making a bold statement," I am not implying that red is making a bold statement in my day, but in his.
To get a better analogy, imagine if I said that Napoleon said that he had a new sword, and by "new" he implies his other sword was old, and what is old is wearing out and already fading away, it is well understood that I am not saying that the sword is becoming worn out and is already fading away in my own day, but in his.
Or to put it in logical terms: X, who lives in Time M, used the word Y in Time M, which implies Z in Time M. What is Z is B and C. If X implies that Y is occurring in Time M, then Z is occurring in Time M. If Z is occurring in Time M, and it implies B and C, then B and C are occurring in Time M.
One can argue that this is only a prophecy of the future and not meant for the returning exiles, and perhaps even exiles, but this would simply ignore the context of Jeremiah and that of Hebrews as well.
This, instead, is the logical implication of what is said since it is Jeremiah, in the sixth century B.C. who says/implies it, and not Auctor in the first century A.D. This also takes seriously the context of Jeremiah that argues this in the context of God restoring the exiles to their land and restoring their living and worship there. And it is also the context of Hebrews where Auctor argues that there is currently no need for any externally communicated covenant since the new covenant has already come and Christ has and is currently fulfilling its role in a far superior way, as the substance of what was to come, than the shadow did in its own day.
What is old in Jeremiah's day is worn out in Jeremiah's day, and is already fading away in Jeremiah's day. When Christ comes, He removes any further need of its remaining elements and takes over the roles those external elements played in the worship of God.
Hence, this is not a time reference that indicates any sort of transition period. As argued before, there is no transition period. The new covenant is celebrated as having already come when Jesus hands the disciples the wine, which symbolizes the new covenant in His blood. This same new covenant is celebrated by the early church in communion, as evidenced in 1 Corinthians 11:25-26. The new has come. The old has faded away as a necessary means to worship God. And this is Auctor's point. The superior has come, so the inferior has already faded away. The renewed and superior is necessary. The old and inferior is superfluous. Trading in the former for the latter, therefore, is foolish.