The following code does what you request. You can bind swap-parens
to whatever key binding you want.
(defvar swap-paren-pairs '("()" "[]"))
(defun swap-parens-at-points (b e)
(let ((open-char (buffer-substring b (+ b 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 b)
(delete-char 1)
(insert (aref to-replace 0))
(goto-char (- e 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 ()
(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"))))