[Zope] Howto Upgrade a product ?

When a zope object based product exists, and this products is upgraded, many times the old format is not compatible with the new format, so the the old objects appears as broken in the ZMI. Is there a way to upgrade easily this kind of objects ? I was thinking on an intermediate object (that's why I asked about if there is a persistent dictionary product in Zope in my last email) -- __o _ \<_ (_)/(_) Saludos de Julián EA4ACL -.-