It seems like GFDL has an optional declaration that is referred to as the "Or later"-clause. Apparently wikipedia opted for that clause allowing automatic migra

It seems like GFDL has an optional declaration that is referred to as the "Or later"-clause. Apparently wikipedia opted for that clause allowing automatic migration of licenses for all content to newer versions of the license. In collaboration with the organisation behind GFDL, they used that clause to re-license everything under CC. I think doing something like this is not somethings that should be taken lightly. you are risking the only asset the contract has: trust. On the other hand: It is good to know, as an author, that you should not accept releasing stuff under a license that has an "or later" clause.