There was a court case that a lawyer argued that the upper should be considered the firearm not the lower. The upper is where all the magic happens and the lower just holds the fire control mechanisms. There is some inconsistency on what is the control parts with some guns being the lower and a gun similar being the upper. I believe the US mainly left it to manufacturers to put serial numbers on gun and where they put serial number that was control part. In Europe the FNC upper is the firearm and here it's the lower. That's a main reason it's hard to get replacement parts for some weapons here.
I don't think they will require anything on existing guns but in the future they might require upper & lower serial number marked. Considering an AR and people changing out uppers and lowers I think they would have to be 2 separate serial numbers or it would be confusing. The worse case would be upper & lower would be married on new guns with matching serial numbers and existing pre-bull crud law lowers could be matched with older non serialized uppers
I don't think they will require anything on existing guns but in the future they might require upper & lower serial number marked. Considering an AR and people changing out uppers and lowers I think they would have to be 2 separate serial numbers or it would be confusing. The worse case would be upper & lower would be married on new guns with matching serial numbers and existing pre-bull crud law lowers could be matched with older non serialized uppers