Suppose a copyrighted piece work is released under a license like Creative Commons, then the copyright owner changes its mind, could they "undo" it? Could the same work be re-licensed under different conditions? And, what effect does that have on people who are already using that work under the previously published license?
For the sake of preciseness I will give a concrete example:
Let's say a creator releases a video under a permissive license like CC-BY 4.0. You then create a derivative work using this, you provide proper attribution but you publish it elsewhere without pursuing a commercial intent. (i.e. you don't violate any of the CC license terms in place at the time you worked with this).
After some time, said creator contacts you and wants you to refrain from using said video. He says he backtracked on that and now the work is under a new set of terms. Is that allowed?
(not the main question, but also important)
How can one prove that such piece of work was released under said license some time ago?