replace katakana characters with hankaku katakana characters.

ktai-hankaku.el

(defun ktai-hankaku-katakana-region (start end)
  (interactive "r")
  (while (string-match
          "[０-９Ａ-Ｚａ-ｚァ-ンー：；＄]+\\|[！？][！？]+"
          (buffer-substring start end))
    (save-excursion
      (japanese-hankaku-region
       (+ start (match-beginning 0))
       (+ start (match-end 0))
       ))))

(defun ktai-hankaku-katakana-buffer ()
  (interactive)
  (ktai-hankaku-katakana-region (point-min) (point-max)))