Under the construction of the FDL 1.3 we've seen, it might be feasible to have a clause which amounts to "I also agree to licensing my edits under CC-BY-SA 3.0 in the event that this wiki will be migrated to CC-BY-SA 3.0 in the future". Mike, jump in if you disagree, but that kind of agreement would seem sufficient to me to ensure that content can be safely migrated in the future.
The only point here is to not block a clean future migration of new wikis.
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Of course IANAL and all, but it sounds to me like you want to license changes (translations) and not versions. That sounds quite complicated to me. To get things clear, please let's take an example.
Say I am a 64 year old woman from the Qurinyi tribe in middle Africa (dont look it up, it does not exist). And I just want to start the Qurinyi Wikipedia with my recent skills to type together with my friends on the new computers we just got. I speak well French, so a logical start would be to start translating major articles at least partially from French.
Would it be OK to do that in the new situation? Strictly speaking, there is still (GFDL) copyright of the original French authors on the text, but on the original there is no CC-BY-SA license. Am I or am I not allowed to add a CC-BY-SA here? And if I would only say that my part was cc-by-sa, how can I identify what is mine then?
Thanks for clarifying. If it is all ok, sure, no problem for me. I just would like things not get extra complicated for people in these new wiki's :)
Best regards,
Lodewijk