Absolutely can be done. The whole point to the designers of the 6.8 was to put a more effective round into the existing AR platform with a minimum of fuss. Barrel, bolt, mags.
A couple of weeks ago, I would have said that you should probably just get a 5.56 upper and keep your 6.8 intact, but you will probably find that a tad more expensive now than buying those few parts you need and rebuilding your upper.