This test is still in use today.
Ex post facto has a relatively narrow legal interpretation in the U.S. - essentially, it only means you can't *increase* punishments in a criminal case retroactively. Decreasing punishments is (and should be) fine.
See Calder v. Bull. Note that compulsory sex offender registration is not considered a punishment by SCOTUS.
What this country needs is a good five cent nickel.