As animuson wanted this phrased as a question:
Are you sure you're legally allowed to relicense all content without asking the copyright holders (i.e. all contributes) individually for permission? If so, by what justification? As detailed in other answers it looks like CC-BY-SA 3.0 doesn't allow it.
The question has been asked before, but never answered definitively.
The previous relicensing plan explicitly did not apply retroactively.