The procedure is quite simple in principle: each new contributor confirms their permission to use their code under two licenses—MIT and GPL3. After a while, the entire codebase becomes dual-licensed. At that point, I leave it as is and (tentatively) copy the MIT portion to another location. Yes, the derived work will still remain under the dual license, and therefore under the GPL3 as well, but no one can stop me from copying the MIT portion, which, by coincidence, will cover the entire project. This isn't relicensing; it's rewriting it under a dual license.