class SomeClass:
    def __init__(self):
        self.target = 'world'

    def hello(self):
        print('hello {}'.format(self.target))


some_instance = SomeClass()

class ProxyClass(some_instance.__class__):
    def hello(self):
        print('hi {}'.format(self.target))

some_instance.__class__ = ProxyClass

some_instance.hello()