A comparison of the Qumran manuscript of Isaiah with the Massoretic text revealed them to be extremely close in accuracy to each other: "A comparison of Isaiah 53 shows that only 17 letters differ from the Massoretic text. Ten of these are mere differences in spelling (like our "honor" and the English "honour") and produce no change in the meaning at all. Four more are very minor differences, such as the presence of a conjunction (and) which are stylistic rather than substantive. The other three letters are the Hebrew word for "light." This word was added to the text by someone after "they shall see" in verse 11. Out of 166 words in this chapter, only this one word is really in question, and it does not at all change the meaning of the passage. We are told by biblical scholars that this is typical of the whole manuscript of Isaiah

From this example, 1000 years passed and 17 letters changed from the original manuscript, without affecting the meaning. I suppose it's possible that there are places in the old testament that have been altered slightly over generations, but that's not enough to question the integrity of the manuscript. You also can't assume they're making copies of copies of copies, eroding like a game of telephone. Several generations might have worked off the same manuscript over hundreds of years.