So this is my "opinion" based upon decades of working with ATF.
If you have 1 SBR, you can have as many uppers as you want. You can have a dozen. The gray area seems to be having multiple short uppers and some other unmounted lowers. So if you have 1 SBR and a dozen LE6920s and other complete ARs you are fine. If you have a dozen short uppers hand half a dozen M4 (6920) lowers ATF might look at you sideways.
As far as changing the stated configuration, if you go from 10.3 to 12.5 and you are going to get rid of the 10.3 upper then just update your Form 1/4. Otherwise I'd keep the 10.3 just so you have a registered configuration that is current with ATF. NFA branch is usually pretty cool and pretty informed, they aren't the door kicker type HOWEVER they like for all the records to be correct and current which really isn't a lot to ask.
They really couldn't care if your SBR is 10.3, 7.5, 12.5 or whatever, but WILLFUL non compliance does bother them. If you are going to own an SBR it comes with a certain level of responsibility and a stated correct configuration is one of them.
Personally, uppers don't have tons of resale value and you are probably gonna lose every time you sell one so if you have a 10.3 (especially if it's from a quality manufacturer like Colt, BCM, etc.) I would just retain it and have a selection of uppers that you feel you need.
It's hard to be a ACLU hating, philosophically Libertarian, socially liberal, fiscally conservative, scientifically grounded, agnostic, porn admiring gun owner who believes in self determination.
Chuck, we miss ya man.
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