[Zope] Zope 2.7.3b3 Change

I am about to upgrade from 2.6.3 to 2.7.3b3. I note in the change log -- o Make TransientObjectContainer __setstate__ actually work. Upgrades should be flawless from Zope 2.7.0, 2.7.1, and 2.7.2. Upgrades from 2.6.3 will work, but data may not be retained. I am not sure what the implications are here. Can someone explain?