I think there's something fishy about extending the copyright date of stuff that was created before the change, but it's probably not covered by the "no expostfacto" rule, because as several people have mentioned, the works that are affected get extended before their copyrights actually expire, or fall into the public domain and stay there.
But I'm not a lawyer and have no intention of trying to become one or pretend to be one or even play one on TV.