It depends on the law and the nature of the change. As a general rule, once you’ve committed a crime a later change in the law is not relevant. The fact is that you’re still guilty of the conduct that constituted the crime at the time that it happened. That having been said, in some cases if the law is later found to be unconstitutional then it will apply to prior convictions, although that's a rare occurance. At this point, you should consult directly with a local criminal law attorney woh can best advise you further.