"thy law has been burned, and so no one knows the things which have been done or will be done by thee"

Another point in time when the Biblical text was incredibly vulnerable to change and modification was the Babylonian capitivity. The Temple had been destroyed. Fundamental religious institutions had been disrupted. And knowledge of the law was essentially gone. In that time of "Jahiliyyah", Ezra had a mission to reform his society by rewritting the scriptures.

As we read in 4 Ezra (2 Esdras in many English translations), chapter 14:

[20] For behold, I will go, as thou hast commanded me, and I will reprove the people who are now living; but who will warn those who will be born hereafter? For the world lies in darkness, and its inhabitants are without light.

[21] For thy law has been burned, and so no one knows the things which have been done or will be done by thee.

[22] If then I have found favor before thee, send the Holy Spirit into me, and I will write everything that has happened in the world from the beginning, the things which were written in thy law, that men may be able to find the path, and that those who wish to live in the last days may live."

[23] He answered me and said, "Go and gather the people, and tell them not to seek you for forty days

[24] But prepare for yourself many writing tablets, and take with you Sarea, Dabria, Selemia, Ethanus, and Asiel -- these five, because they are trained to write rapidly;

[25] and you shall come here, and I will light in your heart the lamp of understanding, which shall not be put out until what you are about to write is finished.

[26] And when you have finished, some things you shall make public, and some you shall deliver in secret to the wise; tomorrow at this hour you shall begin to write."

[27] Then I went as he commanded me, and I gathered all the people together, and said,

[28] "Hear these words, O Israel

[29] At first our fathers dwelt as aliens in Egypt, and they were delivered from there,

[30] and received the law of life, which they did not keep, which you also have transgressed after them.

[31] Then land was given to you for a possession in the land of Zion; but you and your fathers committed iniquity and did not keep the ways which the Most High commanded you.

[32] And because he is a righteous judge, in due time he took from you what he had given.

[33] And now you are here, and your brethren are farther in the interior.

[34] If you, then, will rule over your minds and discipline your hearts, you shall be kept alive, and after death you shall obtain mercy.

[35] For after death the judgment will come, when we shall live again; and then the names of the righteous will become manifest, and the deeds of the ungodly will be disclosed.

[36] But let no one come to me now, and let no one seek me for forty days."

[37] So I took the five men, as he commanded me, and we proceeded to the field, and remained there.

[38] And on the next day, behold, a voice called me, saying, "Ezra, open your mouth and drink what I give you to drink."

[39] Then I opened my mouth, and behold, a full cup was offered to me; it was full of something like water, but its color was like fire.

[40] And I took it and drank; and when I had drunk it, my heart poured forth understanding, and wisdom increased in my breast, for my spirit retained its memory;

[41] and my mouth was opened, and was no longer closed.

[42] And the Most High gave understanding to the five men, and by turns they wrote what was dictated, in characters which they did not know. They sat forty days, and wrote during the daytime, and ate their bread at night.

[43] As for me, I spoke in the daytime and was not silent at night.

[44] So during the forty days ninety-four books were written.

[45] And when the forty days were ended, the Most High spoke to me, saying, "Make public the twenty-four books that you wrote first and let the worthy and the unworthy read them;

[46] but keep the seventy that were written last, in order to give them to the wise among your people.

[47] For in them is the spring of understanding, the fountain of wisdom, and the river of knowledge."

[48] And I did so.

So it seems like the Bible is saying that previous scriptures were miraculously re-revealed after being lost. But another possibility is that the texts were redacted at this point.