Per ATF instructions on my paperwork, if I make a permanent change to an SBR firearm, I'm supposed to send them a letter.  The unit in question isn't an AR15 and it really is a permanent change in BBL and OAL, that I can't so readily change back.   So, on that note I sent them their letter with my new dimensions.   What happens next?  Do they update my registration paperwork with that somewhere?  

I ask, because I keep a copy of my paperwork in the gun-case, and I suppose it's no longer current, at least in the description section.  Yes, I know this is highly academic and nobody cares.  But all the same, curious how this works, and if my official paperwork gets updated on their end, or otherwise shows up somewhere official?

They update the registry on their end, and you get no new paperwork or anything. You may or may not get a response letter simply saying they acknowledged the change.