XML.com has published my article on a theory of compatible versioning at http://www.xml.com/pub/a/2006/12/20/a-theory-of-compatible-versions.html.
The key sentences are "This article describes a set-based model for explicit extensibility and understanding extensions that maximize the versioning capabilities of any language, including languages defined by XML Schema or other XML vocabulary formalisms. Using some simple set theory, we will show that providing extensibility in the first version of a language is the key to compatible evolution."