Common Lisp the Language, 2nd Edition

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### 28.1.10.3. Customizing Class Redefinition

Methods for `update-instance-for-redefined-class` may be defined to specify actions to be taken when an instance is updated. If only `:after` methods for `update-instance-for-redefined-class` are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of `update-instance-for-redefined-class`. Because no initialization arguments are passed to `update-instance-for-redefined-class` when it is called by the system, the `:initform` forms for slots that are filled by `:before` methods for `update-instance-for-redefined-class` will not be evaluated by `shared-initialize`.

Methods for `shared-initialize` may be defined to customize class redefinition (see section 28.1.9.5).