Yes, and if that conflicts with what the courts believe the law has always meant, those decisions won't survive contact with the legal system. An ex post facto law is a law creating (or enhancing) criminal penalties for acts that exist before the law is passed. Changing administration interpretations are like changing prosecutorial priorities (and the former comes with a lot more notice and specificity than the latter) -- they only have effect so long as they are within the bounds of what the courts will accept was covered by the law when it was passed.