Textual: The first person of Zeniff's account is now replaced with a very clearly synoptic third person account. Where up to this point Mormon entered Zeniff's record as he found it, now Mormon abandons copying and turns to abridging. Why does he make this shift in editorial method? Zeniff's account is succinct and informative. It provides important information without providing too much information.
The account of Noah is likely to have been longer. As a more established kingdom, and particularly as evidenced from Noah's excesses, we may see him as a fairly self-important king. Such a man would make sure his record was impressive. In addition to length, however, Mormon would be completely unimpressed with Noah. Mormon's introduction to Noah begins unflatteringly, which is certain to be a contrast with the tone of Noah's official record. Mormon shifts to telling us about Noah because the original records would not tell of the real Noah.