For (2) you need users consent to that. That generally means, such provision must be reasonably visible and explained to user when he registers or starts editing. Linked post had shown the only thing user seems to consent before she could start editing to is that she's over 13 years old.
Edit: Oh, also. Everyone, please don't downvote parent post. While one may well disagree with what it says, that post is certainly relevant to the discussion, so pushing it down the comments page isn't a good idea. Even if what said there is plain wrong.
You are right that it's not enough to have ToS; the users should actively accept them. But what was plain wrong about my post? It said typically this is how it's supposed to work, in which the wiki has a right to change its licensing. Not that TVtropes ToS is like that or that what they did was right.
If the only provision of the ToS that you're accepting is that you are over 13, then the wiki has no rights to even distribute your content under CC-BY-SA. So the conversion to NC isn't even relevant.
And in any case, you own copyright to anything you author, unless you expressly transfer them to someone else in exchange for some sort of compensation. Absent that, the blogger of OP is fully able to release his work under any license he chooses. And he did that on his blog. So what's wrong?
Edit: Oh, also. Everyone, please don't downvote parent post. While one may well disagree with what it says, that post is certainly relevant to the discussion, so pushing it down the comments page isn't a good idea. Even if what said there is plain wrong.