Talk to me like I’m a dummy.

I registered incomplete lower (no brace or stock just a buffer tube) that I build into an AR pistol. If I was to SBR it, since it was originally registered just as a incomplete lower could I revert it back to a pistol by putting a brace on it when crossing state lines and then have a stock on it while in MD. Or once it’s an SBR it stays as a rifle?