Ok. What happens if GNU decides to Fork the GPL v3 and call it "GPLv3+NSA Exception" and then relicense all their code to this new forked license?

A clause in this forked license is that the NSA can't use it.

That's an addition, not an exception. An exception would say "you must the GPL, unless you are/are doing xyz, in which case you can use abc license". You are saying "you can use the GPL, unless you are/are doing xyz, in which case you can't". That clause cannot legally be inserted in the GPL.

You cannot apply license _retroactively_. Once you release code version under a certain License, the License is associated with that version of the code.