It's at least somewhat unclear whether relicensing can happen anyway even in the absence of a CLA. This question came up a bit at the time of GPLv3 introduction when there was some question whether Linux could relicense even if Linus wanted to--which he didn't. Lawyers like Eben Moglen argued that there were legal theories under which it might be possible. [1]

[1] https://www.cnet.com/news/linux-to-gplv3-a-practical-matter-...