You'd better as hell request an explicit permission to distribute the code from any contributor to your code base, and clarify in the post forms the conditions under which any contribution can be used.
what they actually did was contribute to a codebase - a codebase under my control, and one that I can slap any which license on that I like.
Utterly wrong. Under copyright laws, you can only relicense content that you created, or for which you've been given explicit ownership permissions; if Somebody gave you the code only under the original GPL and didn't assign copyright to you, in order to relicense the code you must first remove any such contribution, so that the result only contains the parts you wrote - otherwise, you'll break their copyright.
This is what is going on in both wikis - the only license under which they published their work at first was the CC-BY-SA (or CC-BY-NC for some Wikias), which is the reason for the sites becoming popular in the first place as many users wouldn't bother to contribute under more restrictive licenses; and neither site requested ownership rights until recently.