> I guess there is no perfect way to do it (for example what if f2 needs some variables defined in Class.__init__), but the situation is the same for Lisp with its dynamic typing.

CL has the generic function update-instance-for-redefined-class[1] for this, so you can customize what happens to instances when a class gets redefined. I think most developers don't actually define methods for it all that often, but it's there if the defaults don't work for you.

[1]: http://www.lispworks.com/documentation/HyperSpec/Body/f_upda...