How to create a subclass instance from a superclass instance

I would like to create a subclass instance from a superclass instance in Python. Suppose I have something like this:

class A():
    def __init__(self, type):
        ...
        self.type = type # this will be something that corresponds to either B or C

class B(A):
    def do_something():
        # this method is subclass specific

class C(A):
    def do_something():
        # this method is again subclass specific

I have a function that receives an instance of A, and I need to create an instance of of either B or C (but there are more subclasses in reality) based on what A's attribute type is. I'm not sure how to go about this. Is there a way out of this or does the solution need to be redesigned?

I should mentioned that I'm working with python 2.7 on this issue.

Thank you