To convert a 6.8SPC to a 223rem/5.56, you can either swap uppers and magazines and just do an "upper swap" that is temporary, or you can swap the bolt (not bolt carrier group), barrel, and magazine to do a more permanent swap.
Depending on the deals you wait for and what you want in your upper, sometimes it's cheaper to find a cheap complete upper than it is to buy a barrel, but more often than not, if you're comfortable swapping barrel (honestly not difficult if you have a torque wrench, armorers wrench, and vice), then you usually buy a barrel cheaper than a complete upper.