I'll probably stick to the perspective expressed to me by the lawyer (I actually consulted on this topic), rather than the opinion of some random "GPL zealot" on the internet.

The procedure described has been done many times, is well described, and is quite feasible, albeit quite complex.

I think I can handle it!

And, in any case, anyone who wants to can sue me in a real court, not an online one.

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.

Hmm. Sounds like it follows the letter of the law (although much of it is LLM-generated, and there’s obviously quite a legal cloud surrounding that), but it does feel a bit against the spirit to me. Overall, I am quite satisfied with the name you’ve given it.