You have filed an ATF Form 1 application make an existing firearm into a short-barreled rifle (SBR). The ATF finally approves your Form 1 application and mails the approval document (the “tax stamp”) to you. Your tax stamp allows you to convert your firearm to the specific configuration of barrel length, overall length, and caliber that you requested, which you then build. Later, you decide to change one of more of the variables of barrel length, overall length, or caliber. What do you do?