If you're talking complete lowers and uppers, it's only a matter of pushing two pins into place. AR-15s are designed to easily break down into a lower and upper reciever without any tools at all. You can swap an upper onto a complete lower in a matter of seconds.
There are "80%" receivers that require gunsmithing, however aren't considered firearms therefore can be purchased without much red tape, and you don't have someone elses name plastered all over the side of your weapon. However, unless you're really into building up your own rifles, that's not for you.
But, yeah, pretty much buy a lower and slap whatever upper you want on there, then change to another upper in about 10 seconds.
ARs are fun as hell to shoot, too. I was trained on an M16A4 and I have loved them ever since.
RRA is a great company, and pretty reasonably-priced. I just built my CAR-15 from a RRA stripped lower and a Colt SOPMOD upper. Great fit and finish.