Having access to a particular instance should certainly not allow one to ask it for its class, and then instantiate the class with arbitrary constructor arguments.
Assuming the Python code in the class itself is not empowered in any special way, I don't see why not. So that suggests that you assume classes can be empowered. I can see this for classes implemented in C; but how can classes implemented in pure Python be empowered?
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)