Comment Re:ex post facto (Score 1) 406
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.
Disclaimer: IANAL