I am getting ready to purchase a stamp to but a sbr upper. I have a AR right now but it is stamped with 556 cal. If i was to sbr this rifle could I use different calibers with a specific registered caliber lower as and SBR? Or do i need to buy a multi cal registered lower and SBR that one? I am trying to buy once and cry once for this route. Next ? is about the trust changes what do they mean by "changes"? I have a trust from a year ago so what do i worry about?