So it looks like he was already a control freak back in 2011, and was attempting to reserve the right to impose retrospectively whatever licence he felt like issuing in the future. I suspect this wouldn't stand up to serious legal scrutiny, but it was already a big red flag before he went off the rails completely.
Correct. You can only change the license for future versions of software. You cannot retroactively change the license for previously released versions.
My understanding of the legal principle at work is called estoppel. (IANAL)
Basically, you cannot prohibit someone from doing something that you already permitted them to do. If you allowed them to use your software before (the previous or current versions) you cannot later go back and say "I changed the license on you." Sure, you can change the license for future versions of the software. From my reading of his website, he is the sole author of the program, so he holds the copyright. He can choose to release the next version under a different license. But you are not allowed to retroactively change the license for previous versions.