I am legitimately curious—and concerned—about the relicensing of answers. It really seems as though they shouldn't be able to do that. And if they can, can they also change the license to "all rights reserved by SE"?
Like GPL, CCBYSA allows you —the licensee— to pick a future version of the same license. That is, if you receive a copy of work under v3, you can make it available under v4, no questions asked.
That's all SE has done here. Exercised a clause in the license. If you don't like that, don't submit content under licenses that allow it.
Edit: I'm misreading the text[0]. Section 4, clause b:
> You may Distribute or Publicly Perform an Adaptation only under the terms of: (i) this License; (ii) a later version of this License with the same License Elements as this License ...
