Alright gents, please bear with me. I’m a pistol guy and have not even owned my rifle for a full year yet. I searched the forum and googled the google. I have found answers to most of my questions, but want to confirm a few things.
I have a DD mid length .556 system right now. I’m planning on just getting a 300 BO upper.
Can I get a “stripped upper”, and then just use my bolt and charging handle from my .556? Does a “stripped upper” consist of the receiver, barrel, gas tube, and hand guard?
Do I need to change anything in my lower to run this? I read on the AAC site that “The 16” AAC 300BLK Upper is designed for use with a carbine receiver extension, standard GI Buffer Spring, and a CARBINE Buffer." I’m not necessarily getting an AAC upper, but does this mean that I need to change the listed components to something other than what’s in my rifle now?
Any and all help will be greatly appreciated.
All you need is a barrel. Everything else is 556
Your google-fu is weak. Stripped means just the receiver: none of the parts in it. It will cost $100+. That AAC 300 upper costs probably $600+, because it has all those parts including the barrel. Unless you are making a permanent change from 556 to 300 (not recommended), don’t swap barrels.
Leave your 556 upper alone and buy another barreled upper…one with a 300 barrel in it. Don’t swap BCGs back and forth or anything crazy. If you have the bones to ramp up to another caliber, do it right. But yes, you can leave your complete lower receiver intact, and simply switch that, depending on which upper you are using.
Yes you can buy a stripped upper and switch your current BCG and charging handle over! I wouldn’t do it if your current BCG has thousands of rounds through it as it would be pretty much worn to your current 5.56mm! Best thing would be is to get a complete upper and just change over to 300 BO when you want to shoot it! 
You can but a complete upper minus BCG/CH as a minimum.
A stripped upper is just a piece of aluminum.