I believe the courts slapped BATF down when they tried to smack Thompson-Center owners with a carbine stock and barrel, AND pistol grips and barrels as having the parts to make an SBR, thus establishing the ability, if not the intent to assemble an SBR. If I recall correctly, the courts held that as long as the owner had the parts to build a legal carbine AND a legal pistol, not just pistol barrels and a carbine stock, then BATF couldn't prove intent to assemble an SBR.
I don't think it would be a stretch to apply that logic to 922(v)(2) exempt (preban) and non-exempt (postban) receiver combinations. In other words, as long as you have a legal preban, you can have an extra preban upper. No preban lower, no preban upper.
Do not take this as legal advice. Consult a knowledgeable attorney for real legal advice.