[Zope] How to add a property to a file system product class ?

Hi everybody, I have a file system product, it has a class with some properties, but I now need to add a new property to the class, it works ok for the new objects that get created, but the old objects doesn't have the new property added, so when I try to display it, zope gives me an error. What should I do to get the new property added to all the old objects ?. thanks a lot, Orlando Sanchez