I received a comment verbally while at OGF21. The gist:

Schema are often part of a -R-P document.  These can be updated separately from the document, without going through a 
formal process.  Versioning should be used to track schema updates.  If a new schema is a necessary fix (i.e., something
 in the older published schema was broken or incomplete), then an errata report might be needed.  The point is that 
going from -R-P to -R might involve a new schema that is not reflected in the -R document.  (This is because doing so 
would probably be too much of a change for for the errata process.)

Also: since such schema updates are likely to be part of work going into Experimental documents, it's OK for these -E 
docs to have an updated schema.

I think I got the gist of these comments correct...  they were received verbally from a WG chair.