True . . . all you would need for the swap is the bbl. Complete BCG and mags are compatible. You might have to do a buffer and spring change depending on what you want to feed it. Or how you want to shoot it.
One "problem" that I had with doing this(for rifle) is that a 16" bbl is(pertty much) your only choice. The complete uppers are coming down in price. Not quite like a 223/5.56 but getting there. I reccomend a bbl with a 1:7 twist also. This will help should you decide on some heavier bullets. There are some 1:10 twist bbls floating around out there. Do not get one of these.
When looking for AR suff nowadays,instead of checking the "normal" sites like Midway,PSA,etc,etc,I google what I am looking for. Sometimes there are good deals on AR stuff from places that you would not normally consider. Always check Sportsmans Guide and Brownell's. --- SAWMAN
If you like the rifle setups you have I'd just swap the barrel. Depending on the gas block the barrel swap may be a little more involved. If you're interested in changing the rail/handgaurd or other parts, complete uppers are coming down.
For just a barrel I'd check Faxon, Aero, Palmetto (FN chf).