replace instances of the word at the cursor (or current selection) with a new word

replace_sel_in_file.el

(defun cole-foo-balls (beg end r)
  (interactive "r\nsReplace sel. with: ")
  (let ((s (if (use-region-p)
              (buffer-substring-no-properties beg end)
            (word-at-point))))
   (progn
     (goto-char 0)
     (replace-regexp s r))))