>The copyright holder only has to provide source to those who got it from them under a GPL license. Others may not be able to get a copy, as those who can ask you for it aren’t obliged to give it to them on request
That's true, but if the software is at all widely distributed, then there's lots of people who have a copy theoretically, and any one of them could distribute their copy. So, in practice, once the cat is out of the bag, it's very hard to get it back in. But if the copyright holder changes the license (even on older versions) and no one still has a copy of those older GPL-licensed versions (or isn't willing to send a copy), then sure, in effect the copyright holder could retroactively un-GPL the whole thing.
That's true, but if the software is at all widely distributed, then there's lots of people who have a copy theoretically, and any one of them could distribute their copy. So, in practice, once the cat is out of the bag, it's very hard to get it back in. But if the copyright holder changes the license (even on older versions) and no one still has a copy of those older GPL-licensed versions (or isn't willing to send a copy), then sure, in effect the copyright holder could retroactively un-GPL the whole thing.