I’ve removed the A2 FH from my 14.5” SBR and replaced it with a Surefire Warcomp.  Since that is more than 16” long, I’ve had John Thomas P&W the FH and I’m putting the upper on a standard title 1 lower.  I’m going to put a 10.5” .375 SOCOM on the SBR lower.  My question, do I need to advise ATF of the caliber and barrel length change?  TIA

They ask to be notified of "permanent changes".  Since it's an AR and capable of such easy changes, the general guidance I've heard at the Holiday Inn is notify them if you can no longer put it in the original configuration - sold the upper/barrel, p/w a new muzzle device onto the barrel, etc...

Edit:  My 9mm SBR was originally configured with a 12" 5.56 barrel, which I still have.  I submitted a Form 20 with the details as currently built, and the ATF approved with caveats of the registered details.

No.

You don't even need to notify them in the event of a permanent change, but...

No. But why would you ever P&W?
You have a Registered lower.
Even if you didn’t... a pistol lower would be the way to go.

Now I have two complete guns.  A 16+” title one and an SBR, instead of just an SBR with two uppers.

Pistols with a “brace” are a non starter with me.  Build a real gun if you want less than 16” and wish to fire from the shoulder.

More options are always a good thing. That’s kind of my point. I’d just build the lower as a pistol first... to keep my options open. YMMV.

And a P&W just closes options, especially upper wise.