Constructive intent. Unlikely to ever get hit with it, but if you don’t own a lower that the non-HBAR upper can be legally mounted on, legally you could be guilty of constructive intent. Get pulled over driving back in to the state shooting with that non-HBAR upper and the trooper decides to search your vehicle for reasons (they can be manufacturered reasons) and finds it, you could be up a creek.
You’d want to leave that upper out of the state. Or the barrel
If you replace just the barrel.
Do piston AR's use the same barrels as DI AR's? If that is the case, it would be possible for a recent arrival in MD to possess a government profile barrel if he/she was building a fully-lawful today piston AR.