As it is now: the proxy would point to a different resource
on a another rr server (after an upload to the new server),
but the old resource on the old server is still there and
can issue auth requests for this resource.

Doing this the 'right way[tm]' would imply a server to
server move of the resource, which is not implemented yet.

A much cheaper way would be to disable the choice after an
upload was done.

Use cases and comments would be nice. :)