Heya, I know you can legally swap out uppers of different calibers on lowers but what if I don't own a caliber/setup that i used when creating the SBR paperwork? I originally filled mine out as a 300blk with 10.5in barrel. I was thinking of selling my barrel and bailing on the 300blk all together and just doing it in 9mm. Legally speaking do i have to own a 300blk upper so I did not lie on my paperwork? Dunno if thats a stupid question or not as they wont come to check my safe for an upper or not.

Just need to send a letter to the ATF stating you made a permanent change to it

You are not required to notify ATF even of permanent changes, though they ask that you do so.

If at some future time you try to transfer the SBR to someone else, and its description no longer matches, you'll most likely be required to document the changes at the time of the transfer, usually with pictures, so that the registration can be corrected before the transfer is approved.

delete.

Just to clarify, you haven't made the SBR yet because the stamp hasn't been approved, but you want to know if you can make the SBR in a form other than originally approved?

That seems different than changing the configuration of an SBR which is already made. Safest path would be to hold your 300 upper until approval, make the 300, then switch to whatever and sell the 300.