One interesting thing you won't find in the XPS spec I pointed to is how to declare that one assembly is a later version of another assembly.  This will be described in the formal xaml spec Rob is working on, but the quick description is you stick the XmlnsCompatibleWithAttribute on your newer assembly, eg:

[XmlnsCompatibleWithAttribute(oldXmlNamespace, newXmlNamespace)]

This is known as "subsumption" (a word I don't believe ever made it into the official spec) -- the new assembly is said to "subsume" the old assembly.