Trivial changes, e.g., those that can occur when doubly loading the same code, do not require updating the instances. These are the changes that do not modify the set of local slots accessible in instances, e.g., changes to slot options :INITFORM, :DOCUMENTATION, and changes to class options :DEFAULT-INITARGS, :DOCUMENTATION.

The instances are updated when they are first accessed, not at the time when the class is redefined or MAKE-INSTANCES-OBSOLETE is called. When the class has been redefined several times since the instance was last accessed, UPDATE-INSTANCE-FOR-REDEFINED-CLASS is still called just once.