I e filed my form 1 for my first sbr build about 10 days ago. I was planning on an 8.5" 300 blackout. I have now decided I would prefer to use a 10.5" barrel. Can I use the 10.5 or am I stuck with the 8.5? I know I can add different uppers to it but wasn't sure if I had to have an 8.5 around since that's what I filed for?

You don't have to keep the upper around you registered the lower with, heck you don't even need to own an upper when you register. The lower is what counts. Once a lowr is a sbr, it will always be a sbr unless you contact the ATF and have it removed from the registry.

A lower is not, and cannot be, an SBR.  A rifle is an SBR when it's configured as one (i.e., barrel < 16" and/or OAL < 26").  Put a 16+" upper on it, and it's a normal Title I rifle.  Remove the upper from it and sell just the lower, and it's normal Title I firearm (and it's lost its NFA status--the buyer of the lower can't assemble it into an SBR configuration without a new approved Form 1).

To the OP: You are supposed to initially build the configuration you specify on the Form 1, but having done that you can change to whatever configuration you like, and there's no requirement to be able to return the rifle to the filed configuration.  You might observe that there's then no way that ATF or any other agency could know whether you initially built it in the specified configuration--and if you don't do something like post it on the Internet, you'd be right.  ATF asks, but has no basis to require, that you notify them of any "permanent" change in configuration.  It's hard to call anything "permanent" with a modular platform like an AR, but if you wanted you could send them a letter after your Form 1 was approved notifying them that you're changing the configuration to a 10.5" barrel.

OP, I had a similar question so I called the ATF. I was puzzled by the idea of the Form 1 asking for OAL.

I was thinking of building a SBR with a standard flash suppressor. Later I thought I'd like to try adding a QD suppressor (silencer)

but that would mean a different FH/MB (flash hider/muzzle break) and would change the OAL length.

Called the ATF and the lady I talked to said the SBR length could not be changed in any way without totally re-registering.

Might have been her "off" day but that's what I was told.

Hence, I'm currently looking at suppressors and their QD FH/MB so I can add the FH/MB I'll need to the initial build and buy/apply for the suppressor later.

FYI only.

ETA: The paper Form 1 has a sheet for additional calibers with barrel length and overall length. Maybe you can e-file an amendment that way? Just a thought

Unless you're permanently attaching the FH/MB (pin/weld or silver solder), it doesn't count in barrel length or OAL.

Maybe you phrased the question poorly, maybe she just misunderstood, but the bottom line is that she's simply incorrect.  You need ATF permission (in the form of the approved Form 1) to make an SBR.  There is no requirement anywhere in the US Code or the ATF regulations to notify, much less ask, ATF for changes in an existing, approved SBR.  You may change calibers, barrel lengths, and muzzle devices freely without restriction or obligation.  It's a widely-held belief that you must retain the original upper, or one matching its length and caliber, but that belief also has no support in the law or regulations.

Plus, what BigWaylon said--muzzle devices don't count toward barrel or overall length unless they're permanently attached.

Thanks for the clarification. Hope it helps the OP out as well.

I had a little bit of a suspicion when the lady was ill-tempered but I thought she'd probably

been having a bad day but would give me the right scoop on legalities.

I guess I can pi$$ a woman off without even trying!