Just flew ATL-CDG, CDG-FCO. That was also a 2C to 2D connection. Unlike the others, they have you walk the 2C to 2D connection. This is after the bus drops you off at passport control (or in the unlikely event you get a jetway upon arrival at 2C, after you walk through passport control).
It's a bit of a hike. And, yes you do have to back through security.