A lot of people have a lot of weird superstitions about the GPL. It's probably useful to read the license sometime. It may also be useful to acquire a basic understanding of copyright law, if you are a software developer.
Long answer: One of the conditions for contributing code to the GNU project is assigning copyright to the FSF. That means they're legally the sole copyright holders for all software they distribute. The official reason for this is so they can pursue GPL violations in court without involving the actual authors of the code. It also permits them to change the license on any of their software. Finally, there are various reasons they might be able to do this anyway even if they didn't have the copyright assignment rule, so long as the new restrictions didn't conflict with whatever license the original software was distributed under.
That's really fucked up. (I trust they won't do such a thing, but I didn't think such a thing could be possible)
So if they licensed it to you as GPL, and then say "From now on, shareware is the way forward!" then you can continue to share yours as GPL, fork it, etc.
But, I think if they were to go rogue it would be a matter of forking a GPL-licensed version of the software and continuing development on that under an open license.
And all this is ignoring that most GNU programs are licensed under a GPL variant and any future version, so they would not need the copyright to upgrade the license anyway.