On the AR, pretty much all the magic happens in the upper receiver, so as long as the sights or optic were pre-zeroed, installation on a lower wouldn't make any real practical difference.
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There is the fit between the upper receiver and the lower receiver that could come into play.This slight and there is available a silicone wedge that can be inserted to remove all play.
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Beat me to it. Upper to lower fit doesn't matter much at all. Even the accu-wedges are more for the OCD portion of the shooting community than anyone trying to get better performance.
The modularity aspect for the individual Big Army troop isn't that great a strength on the AR/M16/M4 for the simple reason that it isn't used as such -- units just don't have additional specialized uppers, etc. However, I know a lot of SF guys, in some cases entire ODAs, that have deployed with multiple uppers for their M4s, and some guys who used SPR uppers on M4 lowers with pretty good success (trigger is not as good as the one on the issue SPR lower . . . but non-match grade M4 triggers never had an army-wide safety warning about match grade triggers failing on the SPRs).
It makes pretty good sense if you're either doing mounted operations and have extra stowage for spare uppers (starts making much less sense when you've hauling a golf bag of gun parts on your back on a ruckmarch) or if your mission set(s) is/are deliberate enough you can preconfigure your weapons -- going into Fallujah, maybe 12" barrel uppers are preferable, patrolling some wide open stretch of nothing in Iraq or Afghanistan, maybe 18" uppers are better.
That said, the bigger show stopper in my experience is ammunition quality rather than optimizing barrel length. With good ammo (Mk 262) and an ACOG, shooting unknown distance targets out to 600 meters gets boring, and (with the aid of a ballistic computer and spotter making wind call) I've seen a guy make a hit on a steel chest plate out around 1200 meters with a 12" upper. With standard issue green tip, you're probably more consistently in the 3-4 MOA (and realistically, anywhere from probably 1.5-6 MOA, depending on lot), which out at 600 which is pretty much just rolling the dice before you even start worrying about things like wind, bullets going transsonic, and such.