Common Lisp the Language, 2nd Edition
The first step modifies the structure of instances of the redefined class to conform to its new class definition. Local slots specified by the new class definition that are not specified as either local or shared by the old class are added, and slots not specified as either local or shared by the new class definition that are specified as local by the old class are discarded. The names of these added and discarded slots are passed as arguments to update-instance-for-redefined-class as described in the next section.
The values of local slots specified by both the new and old classes are retained. If such a local slot was unbound, it remains unbound.
The value of a slot that is specified as shared in the old class and
as local in the new class is retained. If such a shared slot was
unbound, the local slot will be unbound.