[Zope] Adding properties to a ZClass with existing instances

[R. David Murray]

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