Only thing that matters is permanent changes. If you retain the ability to put it to the originally approved configuration then nothing is needed. If you make a permanent change such as selling the approved configuration upper then the ATF suggests you contact them to make note of the change. 3 locations are permissible; frame, receiver or barrel. My AR's are marked on the barrel. When the short uppers are on the lowers they are marked in accordance with appropriate CFR's. When a 16+" upper is on one of the lowers, there isn't any markings since it isn't in a NFA configuration. Downside is that I can't borrow someone else's upper or build a new one without engraving. I intend to have the lowers marked but in no hurry.