It might not hurt to keep all your receipts for parts to prove the rifle was completed before the ban. Who knows? The lower's serial number proves when it was manufactured but it wasn't a complete firearm at the time. I really don't know how a new law would look at an AR build. Receipts might help.
Of course complete, factory assembled ARs get new uppers and other parts. With all the upgrades done to ARs, who can say when it is completed!