The second step of the update initializes the newly added slots and performs any other user-defined actions. This step is implemented by the generic function update-instance-for-different-class. The generic function update-instance-for-different-class is invoked by change-class after the first step of the update has been completed.
The generic function update-instance-for-different-class is invoked on arguments computed by change-class. The first argument passed is a copy of the instance being updated and is an instance of the class C_{from}; this copy has dynamic extent within the generic function change-class. The second argument is the instance as updated so far by change-class and is an instance of the class C_{to}. The remaining arguments are an initialization argument list.
There is a system-supplied primary method for update-instance-for-different-class that has two parameter specializers, each of which is the class standard-object. First this method checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared as valid. (For more information, see Declaring the Validity of Initialization Arguments.) Then it calls the generic function shared-initialize with the following arguments: the new instance, a list of names of the newly added slots, and the initialization arguments it received.