A functional approach:

      (defun mirror (tree)
        (when tree
          (tree 
              (tree-data tree)
              (mirror (tree-right tree))
              (mirror (tree-left tree)))))

For completness, here are the definitions:

      (defpackage :trees (:use :cl)
                  (:shadow #:copy-tree))
      (in-package :trees)
      (defstruct (tree (:constructor tree (data &optional left right))
                       (:type list))
        data left right)

As well as a test case:

      (mirror (tree 4 
                    (tree 2
                          (tree 1)
                          (tree 3))
                    (tree 7
                          (tree 6)
                          (tree 9))))

      => (4 (7 (9 NIL NIL) (6 NIL NIL)) (2 (3 NIL NIL) (1 NIL NIL)))