change-class

(change-class instance 'class-name initarg value ...)

Changes the class of an existing instance in place and returns it: the object keeps its identity (every other reference sees the change), the slots the old and the new class share keep their values, the slots the new class adds are filled from their :initforms, and any supplied initargs are stored on top. The class argument is a class designator: a literal quoted name, a runtime symbol, or a class metaobject — (change-class obj (find-class 'c)) works, as does a class name held in a variable.

Out of scope: the MOP protocol around it (update-instance-for-different-class is never called), and changing between classes of unrelated inheritance chains keeps the slot values positionally instead of matching them by name.