This is not really accurate:

>>> class x(int): pass
... 
>>> class y(object): pass
... 
>>> x().__class__ = y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment: 'x' object layout differs from 'y'
>>> y().__class__ = x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment: 'y' object layout differs from 'x'