AFAIK, no. I don't see a reason to change an existing registration in any
case -- when we've made changes to the media types system in the past
we've grandfathered old registrations that don't "fit" the new rules.
Not quite the issue. VRML, which is not XML-compatible, is application/vrml.
If VRML is reformulated to be XML-compatible (in a new version), do we
then register application/vrml-xml? I think we do.
Sure, and this is entirely consistent since what you're describing is a new
media type. (An old VRML handler certainly won't be able to process it...)