There's actually some ambiguity over what it would take to relicense Linux. This...

There's actually some ambiguity over what it would take to relicense Linux. This came up at the time of GPLv3. Torvalds didn't want to relicense it so the question was moot but there was some discussion.

Here's a piece from the time quoting Eben Moglen on the topic: http://www.cnet.com/news/linux-to-gplv3-a-practical-matter-n...

(Unfortunately the original linked to article was on a site that no longer exists.) It's just one opinion of course. (And IANAL.)