Trying to deduce what I can from their different website landing pages (all of the upper receiver group landing pages are broken if you search for MK9), I *think* that if you have an older MK9 lower, you would need the conversion kit for the lower AND a new BCG at minimum, for which I can't find a landing page on their website. But the TFB article says: "The fixed ejector has also been done away with as the radial delayed blowback bolt has a plunger ejector". When you look at the graphic on the TFB article, it says "Uses standard CMMG MKGS 9MM upper group".
Combine that info with the information on the landing page for the lower parts and here's how I interpret the requirements:
1: Newer CMMG 9MM barrel "Barrels that are stamped “9mm Luger” will be compatible with an upgraded Mk9 lower receiver. If your barrel is marked “9x19 NATO”, you will need to purchase a new 9mm barrel in order to ensure reliable function."
2: Specific RDB BCG: THIS IS WHAT I CAN'T FIND
3: Lower parts kit that includes mag catch, bolt catch, carbine buffer assembly, carbine action spring, feed ramp.
I assume they will offer a barrel/bcg package like they do for the other 9MM platforms, but it is not available yet. Or it is and they just need to update their site saying that it will work.