gistfile1.txt

In [31]: class A(object):
    ...:     @classmethod
    ...:     def show(cls, x):
    ...:         print(x)
    ...:

In [32]: a = A()

In [33]: A.show(1)
1

In [34]: a.show(1)
1

In [35]: import types

In [36]: def newshow(cls, x):
    ...:     print("You passed {}".format(x))
    ...:

In [37]: a.show = types.MethodType(newshow, A) # 2nd param class -> classmethod!

In [38]: A.show(1)
1

In [39]: a.show(1)
You passed 1