I don't think getting involved in the case would be a waste of time, but from my non-lawyer understanding of ex post facto laws, I don't think it would count as ex post facto.
On it's surface, it seems retroactive, but there is an allotted period of time to comply with the law and it has not automatically deemed since you possessed the magazines in the past and that is now a crime(or rather will be a crime when the amnesty period elapses) that you are automatically guilty of the crime.
Hypothetically, if they could prove that you owned a magazine over seven rounds at some point before the law were enacted, and rounded all those people up and charged them with possession because they at one time before the law was in place possessed what is now an illegal item, that would be ex post facto. However making something illegal, then charging people who get caught, going forward from the time of enactment is not retroactive.
IANAL, but that's my take on it.