The important distinction isn't who owns copyright. It is rather that there needs to be a community opposed to the license change and able and willing to do the work.
One could even imagine scenarios like an originally MIT-licensed software splitting into a commercial company offering commercial paid licenses, plus a community (or even the company itself) offering a GPL-licensed fork. Of course one could then still maintain an additional MIT-licensed fork, but if the rest of the community is happy with GPL and all the development just happens there, your MIT fork will "starve"...
One could even imagine scenarios like an originally MIT-licensed software splitting into a commercial company offering commercial paid licenses, plus a community (or even the company itself) offering a GPL-licensed fork. Of course one could then still maintain an additional MIT-licensed fork, but if the rest of the community is happy with GPL and all the development just happens there, your MIT fork will "starve"...