Right. The two things are different despite having the same ultimate effect, since (in the context of Common Lisp) one is a generic operation provided by the la

Right. The two things are different despite having the same ultimate effect, since (in the context of Common Lisp) one is a generic operation provided by the language, while the other is application-specific with no language support and would need a fragile bespoke implementation for every single use. If the discussion was about Smalltalk that provides become: but not CHANGE-CLASS, it'd be a different question.