Nominative case

Another way of looking at it is [ellipsis]
Her mood has changed from good (mood) to bad (mood).
in order to avoid the repetition of the noun [mood], so actually the noun is there after the preposition [from] and [to].

Hope this helps!

2008-05-14 11:42:25 補充：
yes, strange to use [nominative case] because [nominative case] is the case for [subject] not for[object], after [prepositon] it should be [objective case].

2008-05-14 11:47:40 補充：
let me know which grammar book you are talking about and maybe I can read it and see what the author is trying to explain.

2008-05-14 12:21:51 補充：
or may be the book is talking about nominative case and objective case (after preposition), [one] can be used in this case, but since [good (mood)] and [good one] are the same, that is [mood] or [one] as the object, it is unnecessary to repeat it with [her mood] in front as th subject.

The word "from" itself can act individually as a preposition, and that is exactly the case in your captioned sentence.
Her mood has changed from good to bad, or
Her mood has changed from being good to bad
Can you see any difference in the meaning nor any mistake in grammar ?
I cannot see any difference. Plus I believe if we can make a sentence clear in meaning with correct grammar, we should avoid making it complicated.