Several variables exist if I understand your question correctly.
AR15 are considered weapons of war when it comes to import into the USA, so technically speaking a USA made AR15 can not be imported into the USA once it has been exported. This ruling extends to pretty much all rifles that were ever used in any military and in any conflict. Ie Lee Enfields, Carcano's Mausers etc.
The gray area from my experience is whether the BATFE would allow just the upper back into the USA, assuming it had longer than 16 inch barrel. Shorter and it falls under the SBR rule putting it in about the same category as suppressors and F/A.
I think your best bet would be to call Amber as was suggested for clarification.