[Zope-dev] Product upgrades

I have done work on a couple of products and one thing I've noticed is if you change what is done in the __init__ method of an object there isn't a good way to make the change so that it takes effect on old instances of the object. For example: def init(self): self.data.append('cow') becomes: def init(self): self.data.append('cow') self.data.append('horse') things created with the first constructor must be removed and recreated if 'horse' is to be added. Is there a way to go through and update old instances of a product to a newer version? Or should backwards compatibility be designed into the product? Thanks, --Brian