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According to Wikipedia's official copyright FAQ:

According to the WMF legal team, CC BY-SA 4.0 is not backwards compatible with CC BY-SA 3.0. Therefore, mixing text licenses under 3.0 and 4.0 would be problematic, however media files uploaded under this license are fine.

Why didn't Creative Commons make the 4.0 license officially compatible with the previous version? Isn't this making it hard for projects like Wikipedia to reuse CC content in the future?

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    Copyleft licenses require that the work stays under the same license. Later versions of the license are assumed to be better, so the CC licenses also provide an upgrade path. However, they do not support downgrades to earlier versions. This approach works like a ratchet: the work can be moved to a better license, but not to worse licenses. A CC 3.0 work that wants to include CC 4.0 material is free to upgrade its license, thus the licenses are technically compatible. Wikipedia already did one round of license upgrades, from GFDL to CC-BY-SA 3.0. – amon Nov 8 at 21:05
  • @amon the problem is that 3.0->4.0 is impossible for an entire project and 4.0->3.0 is not allowed. So effectively speaking any website using 3.0 will continue to use it forever or resort to questionable relicensing like SE did. – JonathanReez Supports Monica Nov 8 at 21:53
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    No, you're falsely treating a website as a single Work. But a website can be a collection of Works that have separate licenses. The 3.0→4.0 migration is possible when the Work is an Adaptation of CC 3.0 material, no questionable relicensing necessary. You just can't change the license of an unmodified Work. SE could have done the relicensing properly by adding per-post CC license version indicators, and only upgrading existing posts upon substantial edits. – amon Nov 9 at 11:23
  • @Amon asked a follow up question on this: opensource.stackexchange.com/questions/9012/… – JonathanReez Supports Monica Nov 9 at 15:37
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Copyleft means "you can redistribute the work, as long as you stick to the exact same conditions/license". This way the licenses protect the rights granted by the license for everyone. This means that by default two copyleft licenses can only be compatible if they are exactly the same.

In order to make two copyleft licenses compatible the author of the license have to add some explicit "compatibility clause". The most used copyleft license, the GNU GPL, does it with the "or any-later version"-clause. This kind of forward-compatibility makes a lot of sense because normally you want to be able to update to a newer version of the license which most of the time improve the wording and/or take new legal and technical challenges into account.

On the other hand backward-compatibility would make all the improvements of the newer version void because everyone could ignore it and go back to the older version. I would say that this is the main reason why backward-compatibility doesn't make sense. On the other hand forward-compatibility makes a lot of sense and I'm surprised that Creative Commons doesn't have such a clause.

  • Yeah, the lack of forward compatibility seems like a bad move on behalf of CC. Wikipedia is now stuck with 3.0 for a long time as a result. – JonathanReez Supports Monica Dec 1 at 10:53

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