The function change-class can be used to change the class
of an instance from its current class, Cfrom,
to a different class, Cto; it changes the
structure of the instance to conform to the definition of the class
Cto.
Note that changing the class of an instance may cause
slots to be added or deleted. Changing the class of an
instance does not change its identity as defined by the
eq function.
When change-class is invoked on an instance, a two-step
updating process takes place. The first step modifies the structure of
the instance by adding new local slots and discarding
local slots that are not specified in the new version of the instance.
The second step initializes the newly added local slots and performs
any other user-defined actions. These two steps are further described in the
two following sections.