Python object conversion

This does the “class conversion” but it is subject to collateral damage. Creating another object and replacing its __dict__ as BrainCore posted would be safer – but this code does what you asked, with no new object being created.

class A(object):
    pass

class B(A):
    def __add__(self, other):
        return self.value + other


a = A()
a.value = 5

a.__class__ = B

print a + 10