(let ((the-spec (lambda (x) (evenp x))))

  (defun is-it-to-spec? (x)
    (funcall the-spec x))

  (defun change-the-rules (new-spec)
    (setf the-spec new-spec)))

(defun test (x)
  (typep x '(satisfies is-it-to-spec?)))

;; CL-USER> (test 42)
;; T
;; CL-USER> (test 41)
;; NIL
;; CL-USER> (change-the-rules (lambda (x) (oddp x)))
;; #<FUNCTION (LAMBDA (X)) {5403E68B}>
;; CL-USER> (test 42)
;; NIL
;; CL-USER> (test 41)
;; T
;; CL-USER>