my-requote.el

(defun my/requote-with (c)
  "Replace the quoting delemiter of the current quoted string"
  (save-mark-and-excursion
    (progn
      (forward-sexp)
      (insert-char c)
      (backward-char)
      (backward-sexp)
      (insert-char c)
      (delete-pair))))

(defun my/requote-with-single-quote ()
  "Replace the quoting delemiter of the current quoted string"
  (interactive)
  (my/requote-with (string-to-char "'")))

(defun my/requote-with-double-quote ()
  "Replace the quoting delemiter of the current quoted string"
  (interactive)
  (my/requote-with (string-to-char "\"")))