Yes, if it's first assembled as a "pistol", then you can switch back and forth with different uppers/stocks.
If it's first assembled as a "rifle", then making it into a pistol later gives you a "short barreled rifle".
A truly paranoid person such as myself would take some assembled lowers with bare buffer tubes to an Indiana rest stop, slap a pistol upper on them, and then go home after removing the upper. Yes, they're first assembled as pistols, and no, I never possessed an unregistered pistol in Michigan.