Yep, its that easy. As long as you don't want something like a 243 or 308, then you need an AR10 lower to successfully do that.

When I bought my AR I saved up money and bought it in 2 pieces. I bought a Rock River Arms NM lower for around $280, and found a 16" RRA Varmint upper for around $425.

I've been doing quite a bit of emailing around an checking for lowers, I'm going to do it all over again. Still kicking myself for selling my AR, I should have at least kept my lower and just sold my upper.