change-brackets.el

(defvar swap-paren-pairs '("[]" "()" "{}"))
(defun swap-parens-at-points (begin end)
  (let ((open-char (buffer-substring begin (+ begin 1)))
        (paren-pair-list (append swap-paren-pairs swap-paren-pairs)))
    (while paren-pair-list
      (if (eq (aref open-char 0) (aref (car paren-pair-list) 0))
          (save-excursion
            (setq to-replace (cadr paren-pair-list))
            (goto-char begin)
            (delete-char 1)
            (insert (aref to-replace 0))
            (goto-char (- end 1))
            (delete-char 1)
            (insert (aref to-replace 1))
            (setq paren-pair-list nil))
        (setq paren-pair-list (cdr paren-pair-list))))))

(defun swap-parens ()
  "Transform ( to [ and [ to ("
  (interactive)
  (cond ((looking-at "\\s(")
         (swap-parens-at-points (point) (save-excursion (forward-sexp) (point))))
        ((and (> (point) 1) (save-excursion (forward-char -1) (looking-at "\\s)")))
         (swap-parens-at-points (save-excursion (forward-sexp -1) (point)) (point)))
        ((message "Not at a paren"))))