Why is it that during the Dekoboko Arc (Anime episode 275-277) the seiyuu for Sakata Gintoki, Kagura, Okita Sougo, Kondo Isao, Sarutobi Ayame, and Tsukuyo changed?
Sakata Gintoki : Sugita Tomokazu -> Tomatsu Haruka
Kagura : Kugimiya Rie -> Ishii Kouji
Kondo Isao : Susumu Chiba -> Takahashi Chiaki
Okita Sougo : Kenichi Suzumura -> Itou Shizuka
Sarutobi Ayame : Kobayashi Yuu -> Okitsu Kazuyuki
Tsukuyo : Kaida Yuuko -> Hoshino Takanori
Yet, the seiyuu for Hijikata Toshirou was not changed. Why is that? If it was to keep cost low, then why not have everyone voiced by the usual seiyuu?
I'm not aware whether such sex change has happened before in other anime. If yes, then please state what anime and what kind of approach they were using and the reason for such approach for comparison.