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 contributors) 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.
I just saw that in April 2011 you unilaterally changed all content from 2.5 to 3.0 without any legal explanation. I imagine back then almost nobody noticed. It seems this was after a similar accidental change to the terms.
Also asked as a separate question.