They own the copyright, so they're free to do what they want with the code. That's why dual licensing models (like that of MySQL) are possible.

However, code that's been distributed under an open license stays that way. So Microsoft could add a commercial license, and even stop making updates under an open license. But if they do that, then there's nothing stopping the community from moving forward with their own fork of the open source version.

If I recall correctly, the Darwin operating system might be a good example of how that sort of dynamic could play out.