When I decided to build an SBR, I sent my Form 1 in with a caliber of 5.56 mm and a barrel length of 10.5 inches. I had the stripped lower engraved, a Spikes Multi Caliber, and submitted the Form 1 and now have it back approved. However, my thought process has now changed and I am planning to build a 9MM SBR on the same lower with a barrel length of 7 inches. The 5.56 mm has not been built, do not have any of the parts, will not build it. Once I do the new build, do I just sent the ATF a letter stating the SBR is now a 9mm, 7 inch barrel?