Common Lisp the Language, 2nd Edition
A class that is an instance of standard-class can be redefined if the new class will also be an instance of standard-class. Redefining a class modifies the existing class object to reflect the new class definition; it does not create a new class object for the class. Any method object created by a :reader, :writer, or :accessor option specified by the old defclass form is removed from the corresponding generic function. Methods specified by the new defclass form are added.
When the class C is redefined, changes are propagated to its instances and to instances of any of its subclasses. Updating such an instance occurs at an implementation-dependent time, but no later than the next time a slot of that instance is read or written. Updating an instance does not change its identity as defined by the eq function. The updating process may change the slots of that particular instance, but it does not create a new instance. Whether updating an instance consumes storage is implementation-dependent.
Note that redefining a class may cause slots to be added or deleted. If a class is redefined in a way that changes the set of local slots accessible in instances, the instances will be updated. It is implementation-dependent whether instances are updated if a class is redefined in a way that does not change the set of local slots accessible in instances.
The value of a slot that is specified as shared both in the old class and in the new class is retained. If such a shared slot was unbound in the old class, it will be unbound in the new class. Slots that were local in the old class and that are shared in the new class are initialized. Newly added shared slots are initialized.
Each newly added shared slot is set to the result of evaluating the captured :initform form for the slot that was specified in the defclass form for the new class. If there is no :initform form, the slot is unbound.
If a class is redefined in such a way that the set of local slots accessible in an instance of the class is changed, a two-step process of updating the instances of the class takes place. The process may be explicitly started by invoking the generic function make-instances-obsolete. This two-step process can happen in other circumstances in some implementations. For example, in some implementations this two-step process will be triggered if the order of slots in storage is changed.
The first step modifies the structure of the instance by adding new
local slots and discarding local slots that are not defined in the new
version of the class.
The second step initializes the newly added local slots and performs
any other user-defined actions. These steps are further specified
in the next two sections.