[Zope] Adding properties to a ZClass with existing instances

This has got to be an obvious question with an obvious answer, or I'm doing something obvious wrong, but I couldn't find anything searching the Zope site. The question is too generic to get a good hit searching the mailing list archives... I've created a ZClass. I created a couple of instances. Now I've added some new properties to the ZClass, but when I try to get to the manage screen for the existing instances I get an error that one of the new properties is not defined. How do I get the new properties to be propagated to the old instances? I would have thought that that would be automatic, so I'm figuring I did something wrong. I'm running 2.0.1 (about to upgrade). --RDM