Wikipedia was apparently able to do that because the GFDL 1.3 contained a re-licensing clause. So people who licensed their contributions under GFDL 1.2 "or any later version" could be said to have given permission for this switch.

I did not study all the details of this solution, but my intuition tells me that in most circumstances changing a licence, without truly asking permission from each of the individual contributors, is likely to be legally questionable. Someone either gives permission, or he doesn't. It's normally not good enough to derive permission from the absence of an objection, and definitely not from an objection that is overridden by the vote of the majority.

Anyway, I think the vote was used to make the decision, the licensing situation already enabled the switch (possibly that's debatable, but let's suppose it did). Under such circumstances I would say that relicensing would be possible, because permission follows from the original licensing conditions, which the contributor explicitly agreed with.

Interesting, but I'm glad we've chosen the GPL straight from the start.