I've read a ton of posts on conversions and a lot of questions out there, but no one actually stating or confirming 100% that this works, only that it "should" work.
I am here to tell you it does work, it can be done, and it's not a super hard job.
To convert your new NRCH to the traditional RCH you need:
Bolt Carrier (Technically the actual bolt pictured isn't needed, only the carrier)
That's serious it. HOWEVER. I would recommend swapping the entire bolt assembly (the guts of the BCG) if you are trading with an individual.
That means the bolt, firing pin, etc. Those parts are on a certain range of the tolerances FN has chose, but if you have a rifle on one extreme of the tolerance range and mix with parts from a rifle on the other extreme, due to the floating firing pin design, it's possible to have slam fires or other issues. If you're buying parts directly from FN, you're getting "binned" parts in the middle of the tolerance range, so it's not a concern either way.
IN ANY CASE.
I recommend swapping the BCG with all the guns, charging handle, and front plate.
Installation really isn't much different than a regular field strip and clean of the gun. You're swapping your BCG with the sled and all the guts for standard BCG.
The front plate, you must remove the barrel to get to. It may be pretty tight. There's just a couple screws that holds it in place. Be patient. Try a little heat. Use a gun vice. You'll get it. Make sure you torque the barrel screws to 62 in.-lb. when putting things back together.
That's really all there is to it. I've shot at least 200rd through my gun. I had one feeding issue (probably an ammo issue).
I am a big fan of the simplicity and how robust the original design was as requested by SOCOM. I still feel like the NRCH was FN listening to fans (I mean haters) of the SCAR saying the only reason they won't buy a SCAR is because of a reciprocating bolt (AKA they couldn't afford a SCAR)... Not to mention SOCOM specifically requested a reciprocating bolt. If it isn't broke, don't try to fix it? Even just a basic field strip is frustrating dealing with the SLED and latch system on the NRCH. I'm so glad I swapped.
If you're thinking about doing this, post up on Tac Swap or this forum. It seriously took me a day or two to find someone to trade parts with. And if you're on the other side of the fence wanting to go to the NRCH design...before you pay $1,000 for the kit, see if there's someone that wants to trade parts and save yourself some money.