I proposed this question to Allan Lolly, of Allan Lolly and Associates in California. His answer is below:
Generally, ex post facto relates to criminal punishment. In theory, you know the risks of punishment before you do the crime. Suppose for example someone drives drunk and spends 30 days in jail. The law is changed after he commits the offense and now the new law requires execution for anyone caught drunk driving. The principle of ex post facto prevents unfair play because the criminal was not aware of the potential punishment before he committed the crime.
Civil remedies can be changed retroactively for good cause. For example, a visitor from Brazil enters the U.S. with the Zika virus. The law is changed after arrival so that anyone with the Zika virus must be quarantined. This is not unfair because it is a health and safety concern to protect the public from the spread of infection.
Generally, sex offender registries are meant to be a public safety concern and not a punishment. In a recent case, the 6th Circuit Court of Appeals found that the sex offender registry list as applied is a punishment and so violates ex post facto. In analyzing constitutional issues, attorneys generally will look to see what rights are at stake. Some rights are more important than others. These rights are found in the law and are not subjective. For example, rights stated in our U.S. or States constitutions carry the most sacred rights, but there can be other matters of importance as well. If rights are violated, then we look to see how important is the governmental interest when it interferes with someone’s rights. Finally, is the law over broad and not narrowly tailored to target precisely the governmental concern?
In the case of the 6th Circuit, the law was applied in a way that unnecessarily interfered with individual rights at stake, so much so that the effect of the law was essentially to punish and was not manifestly related to a justifiable governmental interest. I would need to reread the case to give a more detailed analysis, but this is the gist.
In the case of the AWA, I might want to argue the AWA violates ex post facto, but more likely I will argue the statute is vague and unenforceable, or the USCIS agency application of statute is arbitrary and capricious. A concepts in constitutional law overlap and there is interplay between them. You go with your strongest arguments first and then follow up with other viable arguments as well.