In the beginning, pop culture wiki TV Tropes licensed its content with the Creative Commons Attribution-Share Alike license for free content.
I suppose the morale of the story is, if you contributed to TV Tropes before summer 2012, you should know they're distributing your content under a license that you didn't give them permission to use.
No, according the summary the original license was not CC-BY, but CC-BY-SA. The SA part explicitly disallows relicensing with additional restrictions. Since NC is an additional restriction, relicensing from CC-BY-SA to CC-BY-NC-SA is not allowed (unless you're the copyright owner or have explicit permission from the copyright owner, of course).