You cannot retroactively relicense code - only new code. Microsoft can just take the last version that's not GPL.

This is why if i were making an open source project, it will start off with GPLv3, with a commercial paid license if anyone wants to avoid the terms of the GPL. That gives the best of both worlds - open access to anyone, and if they want to modify, they must also be willing to contribute in some way.

Absolutely they keep whatever they took under the license they took it. Obviously.

And they better have that well documented. Legal might have something to say about it, for example.

It gives then a headache and it tells them you don't like what they did.

You can totally rewrite your entire git history to make the entire repository GPL.

You have no obligation to continue hosting the MIT version, and you can release a new version of every commit under GPL.

Yep, this is why GPL exist. "But corporations hate GPL" as I can hear there often, well tough luck… GPL is about the rights of the users, not corporations. Another exemple of why open source projects should adopt GPL or AGPL, nothing less.