"Ex post facto laws are expressly forbidden by the United States Constitution in Article 1, Section 9, Clause 3 (with respect to federal laws) and Article 1, Section 10 (with respect to state laws)."
However, in this case, the law does still allow the ownership of ivory. Its primarily commerce regulations that have changed.
The biggest problem really is the shift in attitude of law enforcement agencies. In the past, because of underfunding and several other factors, the cost of chasing down illegal ivory and establishing a case based on scientific proof was just too high, so it was not a priority.
Now, ivory has become a hot button topic so certain prosecutors are seeking out high profile arrests simply to bolster their resume, AND the burden of proof has been shifted to the average citizen ( to prove his innocence) rather than law enforcement proving guilt.
Guilty until you can prove otherwise.