Although the Constitution’s Ex Post Facto Clause, Art. I, §9, cl. 3, prohibits applying a new Act’s higher penalties to pre-Act conduct, it does not prohibit applying lower penalties. See Calder v. Bull, 3 Dall. 386, 390–391 (1798); Collins v. Youngblood, 497 U. S. 37, 41–44 (1990).
Calder must be one of the oldest cases cited on a somewhat regular basis–and it predates Marbury by 5 years.
And not to be outdone Scalia cites Marbury and Fletcher v. Peck.
Because “one legislature cannot abridge the powers of a succeeding legislature,” Fletcher v. Peck, 6 Cranch 87, 135 (1810), a statute is “alterable when the legislature shall please to alter it,” Marbury v. Madison, 1 Cranch 137, 177 (1803)