I'm not sure it would fall under ex post facto either. What is may fall under is the Supreme Court's ruling on banning weapons in common use.
Using my non lawyer logic, if the NY law is overturned on the basis of ex post facto, other laws such as the closing of the machine gun register in 1986 could be trumped on that basis, because they outlawed weapons that were previously legal. That would get the grabbers screaming
.