> Corporations don't have to maintain open source software under open source licenses if, for example, that software was released under MIT or BSD licenses.

If they own the copyright and control the only source code available, they can change the license regardless of whether it is permissive or copyleft currently.

If they do only the second, they can shut down the repo regardless of the license, even if they don't change the license, and unless it's something like AGPL continue internal use.

If they don't control the only archive of source code, then even if they can change the license going forward, other people can continue to distribute and fork off from the last Free version (again, irrespective of whether permissive or restrictive.) Unless they both own the copyright and have the legal power to retract the license offer for the earlier code (contrary to the usual express terms of license grants, which may be possible if it is a gratuitous license, but even if it is may not be fully effective in all cases because of promissory estoppel.)