Strategies for backwards compatibility when using pickle?

[duncan]

I am working on an application that currently uses a simple pickle to
save a project to disk. 

Every time we make a significant change to our object model we tell our
beta users to delete all of their old projects. 

Does anyone have any advice on how to balance the conflicting interests
of the developers who want to keep evolving the object model and users
who need stable project persistance?

It has been suggested that there may be a way to override pickle to deal
with an evolving object model but it would be nice to hear from others
who have tried to handle this problem. 

Thanks

Duncan