From: Jimmy Wales
If we were starting wikipedia from scratch today, I would prefer to create a wikipedia license that says "You can redistribute this content under GNU FDL, CC-SA, or additional free licenses that may be specified from time to time on this page."
But isn't it too late for that?
Why not add a "from <chose your date> the license is changed to...", and ask all contributers to sign on a page if they retroactively agree, and theoretically one could filter out all articles not covered and rewrite those is necessary.
Schewek