Read my idea again, it is very specifically not ex post facto. Copyright extension is fine, but if you wish to make use of it then you have to consider your past actions. If you don't make use of it then you don't have to consider your past actions. As such it is not ex post facto as it does not change any pre-existing legal relationships.
I would note by your definition of ex post facto then any copyright extension as currently enacted is illegal in the U.S.A. because it changes the legal status of relationships that existed before the law was enacted.
For example I might have a copy of a piece of work that I had paid for on the understanding that by now it would be out of copyright, but the extension has changed that legal relationship. Copyright extension can by your definition only be ex post facto for works created *AFTER* the copyright extension is enacted.
Oh and finally not every legal jurisdiction on this planet bans ex post facto laws. So while it might be forbidden by the United States constitution (which I would note could always be amended), those laws don't extend to the entire planet.