Top comment right now points out that Matrix doesn't require copyright assignment, which means they have to get permission from every contributor or rewrite the code in order to relicense.
They don't. They can fork the "apache v2" version and start adding non apache v2 code given they are the primary contributor. If they own the trademark, they can call the new forked version "matrix".
What non having copy right assignment means is that they can't change the license of existing stuff. But they can change the license of future stuff using existing apache v2 licensed code which is permitted.
What non having copy right assignment means is that they can't change the license of existing stuff. But they can change the license of future stuff using existing apache v2 licensed code which is permitted.