(That's why, actually, I believe that sticking a v2-only notice on code
that the author licenced v2+ is a GPL violation - you are adding
restrictions by denying the recipient the choice of licence.)
You might be denying the reciepient the choice of license. But that does not
violate the GPL, since v2+ says: You can use it under the GPL v2, or at your
choice any later option. If I'm using it under the GPL v3, I'm not bound by
what the GPL v2 says and vice versa.
Also note that the GPL only says that you can't take away rights granted "by
this license" (the choice between GPL v2 and v3 is NOT granted by the GPL!).
It does not say that all rights that the author originally granted must be
preserved...