However why not test what you have with zope 2.5.1 and see if it all works? It is trivial to upgrade zope so I dont' really see a problem. Also if you work with python products instead of zclasses and take the time to design them it should take a few minutes at most to resolve changed to a new version of zope if that.
Well, my experience is that it is terribly difficult to have a stable product, it take time, and when you change the platform version (zope), my products will go back to prehistory from a point of view of stability... (new bugs, "strange things", etc...)
why is it difficult to have a stable product? if you write python products and not zclasses there is no reason for a product to become "unstable". just as a data point, i have written python products for zope for years now and i do not recall ever having to apply patches just to make something compatible with a newer version of zope. i write patches or even rewrite products to take advantage of new things in newer zope releases. that's it. jens