You can swap out various SBR uppers on the registered lower, however the lawmen expect you to keep in your immediate posession the original upper assembly associated with the NFA registered receiver so that IF ever audited / or challenged, by an ATF agent (not just any LEO whom lack the Federal authority to do so) you can present the originally configured NFA item to the BATFE agent (agency). To be completely correct with BATFE etiquette one is also expected to send a courtesy letter of notification to the BATFE as to any alternative SBR / SBS configuration you have in your posession for a particular registered lower assembly. Recall from Form 4......"Proof of Registration: This approved application is the registrant’s proof of registration and it shall be made available to any ATF officer upon request. " (ATF Form 4 (5320.4)
Remember it's not like a drivers license or insurance card presentation requirement to a LEO during a roadside examination....it is a Tax Stamp document, not a license!...."made available" can be complied with by affirming "i will send you a fax / email copy of the NFA document to their Regional Offce, (or they can follow you home to where the documents are located) when I get home or whatever". No different than if a State Trooper asked you for a copy of your 2018 IRS-1040 return on the side of the road, you don't carry it with you and even if you did they have no authority to request it. One good reason to keep photo copies on your iPhone! to mitigate the hassle; where Federal law theory meets practicality!