I agree at the point, that everyone should use that license he like.
Btw, the easiest first step would have been: mail every contributor (there are not that many in that case) that provided more then hast some minor fixes and ask for permission. That is a valid way to change the license.
No, I think, that would not work this way, you have to ask every contributor, no matter how big the influence was. And everyone must agree unanimously. It's almost an impossible task.