You can't make the original code not-MIT-licensed, since the author already released it as such (and even they cannot make it not-MIT-licensed). But you can release it (with or without modifications) under another license as long as it lets you follow the MIT license's only restriction (reproduction of the copyright notice).

What do you think being GPL-compatible (for example) means other than that? The GPL requires you to release all parts of the derived work under the GPL, not just a compatible license.

> You can't make the original code not-MIT-licensed

Yes, that's what I was saying.

Actually you are saying different things. Even thought the original code will still be forever available under MIT, you can, for example, sell it under a proprietary license (without changing a line of code) to whoever is willing to pay for that.

Yes you do (well change is the wrong word, you're sublicensing your copy of the code). Otherwise the software would not be proprietary. Windows for example used to ship BSD licensed code, and it would have been a violation of Windows' license to modify that code (even though the original code was under a free software license).

I don't understand which part of this is confusing you -- the MIT license explicitly states that sublicensing is permitted. If you didn't sublicense the code, then users would be allowed to have all the freedoms that come with the MIT license. This would make it impossible to have a proprietary version of an MIT licensed project -- which is something that we objectively know is possible because it happens all the time.

You cannot change the license on someone else's copyrighted work.

> Windows for example used to ship BSD licensed code, and it would have been a violation of Windows' license to modify that code (even though the original code was under a free software license).

Microsoft didn't include source and if I had obtained the BSD licensed source, I could certainly modify it and distribute it (but not the code written by Microsoft). Also, the BSD doesn't have a sublicense clause which is problematic for your first sentence.

> You cannot change the license on someone else's copyrighted work.

The grandparent already said that "changing" is not the thing being discussed here, but "sublicensing", so this is a strawman.