If your firearm was not configured as a preban before the ban, even if the firearm was manufactured before the ban, you can't legally configure it as preban after. If you were the owner of the firearm before the ban you shouldn't have any problem. It's your word against theirs. They have to prove you wrong. If you obtained the firearm from another party after the ban, it gets a little dicey. In this case, you would most likely need some supporting documentation from the individual you obtained the rifle from. Irregardless, it is my understanding that no law abiding citizen has ever been prosecuted for configuring a firearm manufactured before the ban, as evidenced by serial number, into a preban configuration after the ban.