*scratch*.el

(defun fix-double-capital()
  "Go back to last occurence of a 'double capital' at start of word and correct."
  (interactive)
  (if (re-search-backward "\\b\\([A-Z]\\{2\\}\\)" nil t)
      (replace-match (concat (substring (match-string 0) 0 1) ;concat first char
                             (downcase (substring (match-string 0) 1 2))) t) ; with downcased second char
    (message "No double capital found!")))