anchor.diff:  
   ;;; change-class
   
  -(defgeneric change-class (instance new-class &key &allow-other-keys))
  -(defmethod change-class ((old-instance standard-object)
  -                         (new-class standard-class)
  -                         &rest initargs)
  +;; Common implementation for changing std-class types.
  +(defun std-change-class (old-instance new-class &rest initargs)
     (let* ((new-instance (allocate-instance new-class))
            (old-class (class-of old-instance))
            (old-copy (allocate-instance old-class)))
  +    (cond ((sys.int::instance-p old-instance)
  +           (assert (sys.int::instance-p new-instance)))
  +          (t
  +           (assert (sys.int::funcallable-instance-p old-instance))
  +           (assert (sys.int::funcallable-instance-p new-instance))))
       ;; Make a copy of the old instance.
       (dolist (old-slot (safe-class-slots old-class))
         (let ((slot-name (safe-slot-definition-name old-slot)))
  
positive.code:     (apply #'update-instance-for-different-class
             old-copy old-instance initargs)
      old-instance))
  
  (defmethod change-class (instance (new-class symbol) &rest initargs)
    (apply #'change-class instance (find-class new-class) initargs))
  
negative.code:       (error "Superclass ~S of ~S is sealed and cannot be inherited from."
               super class)))
    (setf (safe-class-precedence-list class) (compute-class-precedence-list class))
    ;; Don't allow exotic function classes to be created.
    (cond ((typep class 'funcallable-standard-class)
           (unless (member (find-class 'funcallable-standard-object)
                           (safe-class-precedence-list class))
             (error "FUNCALLABLE-STANARD-OBJECT missing from CPL of class ~S, CPL: ~:S"
                    class (safe-class-precedence-list class))))
          (t
           (when (member (find-class 'funcallable-standard-object)
                         (safe-class-precedence-list class))
             (error "FUNCALLABLE-STANARD-OBJECT present in CPL of class ~S, CPL: ~:S"
                    class (safe-class-precedence-list class)))))
    (setf (safe-class-slots class) (compute-slots class))
    (setf (safe-class-default-initargs class) (compute-default-initargs class))
    (let* ((instance-slots (remove-if-not 'instance-slot-p
  