You can't "switch" licenses, but I think it's perfectly possible to grant an
additional license like that. People will still be able to distribute the
work under the GNU GPL, in your example, but would be able to choose the more
permissive license if they want to.
Of course, I am not a lawyer.
Honestly, I don't see the point, though. As long as we're all using the GNU
GPL and allowing any later version of it than was originally used, we won't
face a compatibility problem. Of course, such an ideal situation doesn't
exist, but I think those cases aren't really worth losing ground to
proprietary software over. As long as copyright exists and has such
ridiculous term lengths as 2 lifetimes (or forever, in practice), we should
take full advantage of it. Who knows? Maybe it could even end up being the
driving force behind ending copyrights altogether.
