Moving class used in pickle

[Jeffrey Barish]

I have a class derived from string that is used in a pickle.  In the new
version of my program, I moved the module containing the definition of the
class.  Now the unpickle fails because it doesn't find the module.  I was
thinking that I could make the unpickle work by putting a copy of the
module in the original location and then redefine the class by sticking a
__setstate__ in the class thusly:

def __setstate__(self, state):
    self.__dict__.update(state)
    self.__class__ = NewClassName

My plan was to specify the new location of the module in NewClassName. 
However, when I do this I get the message "'class' object layout differs
from 'class'".  I don't think that they do as the new module is a copy of
the old one.  I suspect that I am not allowed to make the class assignment
because my class is derived from string.  What is the best way to update
the pickle?  The only thought I have is to read all the data with the old
class module, store the data in some nonpickle format, and then, with
another program, read the nonpickle-format file and rewrite the pickle with
the class module in the new location.
-- 
Jeffrey Barish