It's an ambiguous question that AFAIK/IANAL doesn't have a clear-cut answer. A while after the GPLv3 came out Eben Moglen ventured an opinion that a work like Linux could probably, from a purely legal perspective, change licenses but he made clear that was just his guess:

http://www.cnet.com/news/linux-to-gplv3-a-practical-matter-n...