I'm sorry if this has been posted in the past, couldn't find it.

I am currently waiting on a stamp for a 300 BO pistol I have.
If I were to ever decide that I'd like to swap that rig into a 5.56 for example, what would I need to do as my paperwork already specifies it's a 300 BO?

Not a thing, do as you please.

I can just swap the barrel out as if nothing?!?!

Yep. It'll only ever matter again if you ever transfer your SBR to someone else, and it's in a different caliber at that time.

ATF does request that you notify them if you ever *permanently* change the caliber, but cannot require you to do so, and what does "permanent" really mean on an AR platform, anyway?

Yep.

The caliber and other descriptors on the Form 1 is the configuration you said you wanted to build (make) and that is the approved configuration for the original build.

Once you build the gun per the Form 1 description you can change it anyway you like.  As was said above, ATF Martinsburg asks that you notify them of any permanent changes, but I have personalty been told by Martinsburg that there really is no such thing as a permanent change to a AR15 variant platform.

An examiner told me that if you ever transfer (sell) the gun that the Form 4 should match the Form 1 so you should do a change letter if you are transferring the gun in a different configuration than you originally made or it might cause a delay.

I have heard from other member here that the change was made by Martinsburg as part of the Form 4 transfer.

Bottom line, if you don't intend to every transfer the gun, make all the changes you want and don't worry about it.

I have done change letters and it just confused everyone.  No more change letters for me.