With all respect you are, indeed, completely mistaken. The license is absolutely and 100% irrelevant, wouldn't matter if it was AGPLv3 or full proprietary or BSD or anything else. Like all copyright licenses, the GPL is a creature of copyright law. Copyright governs derivative works by other people. But the owner of the copyright can derive whatever they want from their own IP. A copyright holder choosing to license their IP under the GPL in no way precludes them from switching to licensing under something else, or indeed to licensing under any number of other licenses (including proprietary) at the exact same time ("dual licensing" is indeed one of the various attempted open source business models) to different people.
Again, they can't do so retroactively, so if they begin by offering software under open source license A and later switch to License B, all the code under A up until the moment of switching to B continues to be licensed under A. So someone can take that as a starting point for a different fork under open source license A, or another one that is compatible. But they won't have a right to any new code done under B if it's incompatible.
Again, they can't do so retroactively, so if they begin by offering software under open source license A and later switch to License B, all the code under A up until the moment of switching to B continues to be licensed under A. So someone can take that as a starting point for a different fork under open source license A, or another one that is compatible. But they won't have a right to any new code done under B if it's incompatible.