FWIW: I still think that allowing to change .__class__ on instances of static types if wrong and should be undone. If we want to make this a feature of the language we should have a PEP and the associated discussion to be able to judge and document the possible consequences of such a change. This ticket is the wrong place for such a discussion.
Regarding you module object change: Why don't you propose a change on the object itself instead of trying to monkey patch it via a mod.__class__ replacement ? E.g. by defining a hook in the object to set which then permits whatever modification you'd like to make.