This is not unprecedented, TCC relicensed part of its code by being approved by all authors:

https://repo.or.cz/tinycc.git/blob/3d963aebcd533da278f086a3e...

The interesting part is that the original author is against it but some people claims it could be a rewrite and not a derivative work.

I don't know the legal basis of everything but it's definitly not morally correct toward the original author.