Python change self to inherited class

I have this kind of structure:

class Foo:
    def __init__(self, val1):
        self.val1 = val1

    def changeToGoo(self)
        HOW???

class Goo(Foo):
    def __init__(self, val1, val2):
        super(val1)
        self.val2 = val2

a = Foo(1)
a.changeToGoo()

'a' is now an instance of Foo
now i would like to change it to be an instance of Goo, by using the method "changeToGoo", and add the other value.

How can this be done in Python?

I have tried:

self.__class__ = Goo

but when I check:

type(a)

it's still Foo, and not Goo.