Let's say I have a programme X licensed under the GPL, to which I am the sole contributor and copyright holder. I distribute the binaries for X and provide the source code online as required by the license. Then, I have a change of heart, and want to relicense under a proprietary license. I know I can make future versions closed source, but I come across a problem if I make older versions closed source - I have distributed binaries which come with a license that requires access to source code - and not just that, but it also requires the provided source code be licensed under the GPL! This effectively makes it impossible to relicense the older versions of X, unless...
Can I change the license of binaries that I have already distributed to this proprietary license? The binaries will have been distributed with the text of the GPL and a notice that they are licensed under it.
(This is an entirely hypothetical question.)