I'm not a lawyer, I just read the copyright lawyers who post here, but I don't think this is all that complicated? Once a literal, specific, concrete file is released MIT, you can't "take it back". That specific file will always be MIT. But you can, on a forward-moving basis, license all your own changes to that file however you'd like. The changes (and the changed versions of the file) have, absent some binding contract that says otherwise, their own copyright status.
This is why there is, for instance, OpenSSH: it's the version of SSH that started when the SSH company decided to use a restrictive license on a forward-moving basis. The community just forked at the last permissive version and took over development themselves.
>That specific file will always be MIT. But you can, on a forward-moving basis, license all your own changes to that file however you'd like.
Exactly, that file is then MIT+YourLicense but one can never take the MIT license away (except the copyright holder).
Take away a (not owned) Copyright would be called stealing.
Even the BEER-WARE LICENSE has this sentence:
>As long as you retain this notice you can do whatever you want with this stuff. If we meet some day, and you think this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
I think the original author (or their heirs) can take away that MIT license during a special “copyright termination” window 35 years after granting it at least in the USA.
This is why there is, for instance, OpenSSH: it's the version of SSH that started when the SSH company decided to use a restrictive license on a forward-moving basis. The community just forked at the last permissive version and took over development themselves.
That's how it's supposed to work?