I purchased a Sig 556 pistol a few years ago and registered it via ATF Form 1 as a SBR. I have gotten a little bored with the current set-up and am thinking about swapping the current Sig 556 lower for a Sig 551 lower (probably the new 551-A1 as the original swiss lower is a little too pricey for me). Anyway, since the upper is the registered part, I assume there is nothing wrong with swapping to a different lower? The only thing I can think of that might cause a problem is that the new lower would have the Swiss folding stock, whereas my current set-up has a Magpul UBR (overall length will probably change). Otherwise, the barrel length and caliber will stay the same. I don't plan on getting rid of the original lower. So, do I need to submit anything to ATF? Or, since I'm keeping the original lower is it good enough that I can convert it back into its original configuration? Thanks for any help you all can provide.