Again, as Mr. K and TM told you, it is NOT an ex post facto. You haven't specifically listed what state you are interested in, but let's take Indiana. It matters not to Indiana whether you were released from registration requirements or even if you were required to register in Illinois at all. The requirement depends only on whether you were convicted of a crime substantially equivalent to the ones listed in their statute.

You're obviously thinking of Doe. V Snyder. That decision, specifically applies to Michigan's law, though it probably would be precedent in the rest of the sixth circuit. It wouldn't apply to Indiana, Illinois, or Wisconsin. The sixth circuit comprises the states of Ohio, Tennesee, Kentucky, and Michigan. The problem is that as you were told by the other posters, there's a Supreme Court decision that pretty much says the opposite (Doe v. Smith). It comes down to not that the registration is unconsitutional but how much other sanctions a particular state may impose "after the fact."