(ql:quickload :flow)

(flow:define-node foo ()
  ((port :port-type flow:n-port)))

(defun test ()
  (let ((foo (make-instance 'foo)))
    (print (flow:port foo 'port))
    (change-class foo 'foo) ; => no applicable method with #<unbound>
    (print (flow:port foo 'port))
    NIL))