class First(object):

    def __init__(self, arg):
        self.arg = arg

    def my_arg(self):
        return self.arg


class Second(First):

    def my_arg(self):
        return self.arg + 2


class Third(Second):

    def my_arg(self):
        return super().my_arg() + 4

el = Second(1)
assert el.my_arg() == 3
el.__class__ = Third
assert el.my_arg() == 7