Is ALTER TEXT SEARCH CONFIGURATION PARSER = new_parser really sane?

After starting to document this stuff I'm wondering whether it really
makes sense to change the parser associated with a tsearch
configuration.  The problem is that the new parser might have an
unrelated set of token types, but we don't do anything about updating
the configuration's mappings.

Ensuring sane behavior here would take a whole lot of new code, and
I'm not sure that I see a use-case that justifies it.  So I'm tempted to
take out that particular ALTER capability altogether.  I note that the
corresponding feature of changing a dictionary's template on-the-fly
doesn't exist (though it'd actually be a lot easier to support).

Comments?
        regards, tom lane