CL-USER> (defclass wall () ())
#<STANDARD-CLASS COMMON-LISP-USER::WALL>
CL-USER> (defclass chameleon () ())
#<STANDARD-CLASS COMMON-LISP-USER::CHAMELEON>
CL-USER> (defvar *what-to-become* 'wall)
*WHAT-TO-BECOME*
CL-USER> (defmethod initialize-instance :after ((instance chameleon) &key)
           (change-class instance *what-to-become*))
#<STANDARD-METHOD COMMON-LISP:INITIALIZE-INSTANCE :AFTER (CHAMELEON) {100BC2DD33}>
CL-USER> (make-instance 'chameleon)
#<WALL {100BC35663}>