Reducing the length of existing copyright terms might be considered an ex post facto law. Those are explicitly prohibited by the Constitution.
The Constitution states that copyrights must be limited in length. Retroactively extending the duration of existing copyrights (as opposed to new copyrights) should be unconstitutional, but the Supreme Court said in Eldred v. Ashcroft that it’s fine—wrongly, in my opinion. Correcting them would require an amendment.