Actually, the seiyuu for Misaki is the same: Enomoto Atsuko (Yukino of Kare Kano), who did the opening theme.
Strangely enough, I too noticed that the voice went up a pitch half-way through the series and it was very perplexing! If they changed the seiyuu then I would understand, but it's the same person and I don't understand why her voice would suddenly change. Bizarre.
Talk about the shows we screen at weekly showing/marathon