RR beat me to it... Pre and post ban doesn't mean anything anymore, and the off-set pins to mate the old colt lowers to new uppers (not sure if new colt receivers are still bigger screws, but my old colt lower needs offset screws). Alternatively, older colt uppers require a bushing to be mate up with non-old colt lowers.
All that said, it sounds like you're really just looking to re-barrel your rifle, not that you're looking for a quick change rifle. A new upper will run between $450-800, while a new barrel can be had for $175-300 and changed out either by you or a good buddy with a little more experience in about an hour (or 20min once you get used to it). Get a set of go/no-go gauges and double check the headspacing and you're good to go. A gunsmith will rebarrel an AR for about $60 around here, so that's another option, $250 is a lot cheaper than $600...