It's important to keep in mind that GPL licensing typically references a specific version of the license, or a specific version "or, at your option, any later version". The latter gives the user the freedom to choose whether to use the originally specified, or a newer, version of the license; whichever is more useful in their specific case.
The license a piece of software is licensed under is attached to that software; using the FSF GPL is just a convenience (for both licensors and licensees). The license comes with the software and covers that software, so cannot (legally) be changed by anyone other than the copyright holder for the software in question.
Also, the GPL by itself places limits on future versions of the license. Let's take the wording of the GPL v3 section 14, Revised Versions of this License, my emphasis:
The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.
Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation.
If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Program.
Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.
So, even if a large company were to take over the Free Software Foundation, they cannot validly alter the GPL in such a way that they can force any previously released software to be used under that license only; they can only authorize further licenses. Which is basically the common concept of dual licensing, plain and simple.
Additionally, the GPL is a license, not a copyright transfer. (This is part of the reason why the Free Software Foundation require a separate copyright transfer when you work on official FSF-maintained software.) The license can only confer rights, it cannot transfer ownership. So the original copyright holder may still exercise their right under copyright law to relicense under "GPL v2 or v3" instead of "GPL v2 or any later version". Of course, previously released versions would still be available under "GPL v2 or any later version" terms.
Further, as alluded to above, nothing in the "or any later version" wording forces a licensee to accept the terms of the new license. The licensee is perfectly free to state "I don't like GPLv4, I'm going to keep using GPLv3" and be fully within their rights as long as they comply with the GPLv3 terms.
Some likely effects would be:
- Existing GPL-licensed software is effectively unaffected, because anybody who cares would probably refuse to accept the new GPLv4
- Everybody relicenses under e.g. "GPLv2 or GPLv3", instead of "GPLv2 or any later version" (if a miracle happens, you can always later state "I authorize FooBaz to be used as licensed under GPLv5")
- Microsoft faces some lawsuits for releasing a license calling it the GPL where the changes made are specifically prohibited by the preceding version of the GPL
- Software fragmentation; any updates done by Microsoft would presumably be GPLv4-only, so the free software community can't make use of those, but this isn't much worse than with the BSDs after the FSF's switch to GPLv3+
- Microsoft loses face completely now that they are finally starting to actually come around in matters of free software and open source
Once the confusion settles, the damage is likely to be minor at most.