RE: Changing representations

> > If I've understood correctly, you have described two competing service
> > models for a given URI: (a) one PUT affects all GET media types; versus
> > (b) one PUT per GET media type.  Both seem perfectly valid and seem to
> > me to fill different use cases.  

> A use case for (b) is when it is expensive for the server to generate
> the different media types, and the server is willing to trust the client
> to maintain semantic consistency between the media types.  Certainly
> this is a very rare use case, but nonetheless valid.  

But this case is much better served by the server maintaining separate URIs for each rendering and redirecting the 'main' (un-PUT-able) resource.
Attempting to maintain separate, expensive-to-produce media types through the same URI seems like a disaster, which doesn't match any implementation anyway.