I'm the founder of a wiki so I know some of these legal issues better than most (although, of course IANAL). Typically how this is supposed to work is: Wiki's terms of use say (1) Users own the copyright to their contributions (authors always do; TVTropes appropriating this is not right) (2)Users grant wiki a non-exclusive irrevocable license to do use the content in any way Wiki likes, including specifically a right…
> Wiki is legally within its rights to change the licensing of its content. Would be if they had proper terms of use/service in place. But TVtropes has no terms as far as I can see, which makes the situation whole lot muddier. Basically we are working on an implicit grant of rights to TVtropes from content authors. The disagreement stems on what those rights include.
Their current terms of use also have no bearing on their legal right to change the licensing from CC-BY-SA to CC-BY-NC-SA. That depends upon what terms of use they had in place before the conversion.
Their current terms are: if you contribute, we own (the copyright to) all your contributions. That's not only unfair to users, it may even be illegal because in a contract you have to provide some compensation in exchange for the release of copyright.