A licence like a Creative Commons cc-by-sa is not exclusive, and allows the copyright holder to do what they like with their work. What they can't do is revoke the licence terms from anyone who chooses to exercise them. At best, the copyright holder can request removal of attribution.

In another comment I mention the Geograph project which uses cc-by-sa. For some more background to that, I wrote about the reasons for selecting that licence to ensure long-term freedom of the archive here: http://www.geograph.org.uk/help/freedom

Yes and no. They can not take away rights granted to existing licensees. But the copyright holder can reissue the same work under whatever license they wish, without having to modify anything.

What the comment you replied to point out is that if I release something under the GPL and then assign copyright to you, you can legally decide to relicense that code under a proprietary license so that the people buying it from me don't have to abide by the GPL.

If I don't assign copyright, on the other hand, any such third parties would need my permission too.

Act of Parliament might do it. They did it for Peter Pan, after all.

I wonder if it would stand a chance as a private members bill....

If a project requires that contributors sign copyright over to a central party, that central party can later choose to release future versions under a proprietary license and profit off of others' past work.

A project that does not require central copyright assignment and that receives sufficient outside contributions cannot have that happen, provided it's licensed under something like the (L)GPL.

Copyright paralysis can be a feature rather than a bug.