Mine was

someObject.changeToClassOf(SomrOtherClass);

(or something like that--i'm was long ago).

In Smalltalk, changing an object's class preserves instance variable (attribute) values when possible, silently dropping if not, and gives it an SomeOtherClass's inheritance (and method dictionaries as a result).

It forced me to realize that an instance of a class wasn't permanently coupled to be an instance of the class that instantiated it or even its current class