You cannot put a pre ban upper on a post ban lower. The trick is finding a pre ban lower...which cost a hell of alot more than a post ban lower.
I will say it again to keep anyone from getting into big trouble down the line: you
CANNOT just put a pre-ban upper on any pre-ban lower. The lower had to be assembled as an evil assault rifle BEFORE the ban took effect. Now, I am pretty certain that the BATFE can figure out if a lower was sold as a complete, assembled evil assault rifle before the ban, but I am not too sure about how they would know if a lower that was sold as a stripped lower before the ban was actually assembled before the ban. Knowing the government though, I would not be quite certain that you will be presumed innocent until they
prove that the lower was not assembled into an assault rifle before the ban.
I am not trying to make Ed paranoid or anything, but let's look at a hypothetical scenario:
Ed buys a pre ban lower and puts a pre-ban upper on it. For some reason Ed's "pre-ban" rifle is noticed by the police during an encounter, either at the home or at the range or wherever suits your imagination. Due to the nature of the encounter, Ed's rifle is taken as evidence. It turns out that the BATFE can prove Ed's alleged pre-ban lower, while manufactured before the ban, did not ship from the manufacturer until after the ban, and it shipped as either a complete lower or a stripped lower, NOT a complete evil assault rifle. Due to the timeline, it is very clear that no purchasing consumer could have assembled a complete evil rifle on the lower before the ban took effect. The fact that Ed put a pre-ban upper on this lower, despite the lower being manufactured before the ban, now makes Ed a criminal regardless of the outcome of the original encounter. Ed is now screwed because he took advice off the internet about pre-ban lowers.
In conclusion, if you buy a "pre-ban" lower make damn sure it was assembled as an assault rifle before the ban. If you cannot be certain that it was a complete, assembled assault rifle before the ban, well, buyer beware, because I am pretty certain that the BATFE doesn't consider "but I didn't know" a valid reason to have assembled an illegal firearm.