This forum post outlines a 2E to 2C trip:
- Disembark and follow the signs for "Correspondances/Flight Transfers."
- You will pass some info desks. These agents tend to be very helpful and should be your first stop if you have questions and/or need onward boarding passes printed. Greet them with "Bonjour," switch to English, and they'll generally treat you well.
- Walk on to the bus stop, clearly marked as "2A/2B/2C/2D/2F". There will be a listing of upcoming buses on a monitor. Just get on the first one that shows a stop at 2C. Don't bother waiting for the express, as it's the difference between 0 and 1 stops. There's an agent at the bus stop who kind of tries to look at people's boarding passes and get them on the right bus.
- Get off at 2C. Like I said, it's either next stop or 1 stop later. You're not even on the bus 5 minutes.
- Pass security.
- Walk up a half-flight of stairs, and you're standing in 2C. Don't get your rollaboard-bag all ready to roll, as you really do need to walk the half-flight of stairs. There's prob an elevator somewhere, but I didn't see it.
Another post in the other direction mentions explicitly you need to pass security:
take the airside bus that goes from 2C, to 2A, 2B, 2F and then S3, one 2E's boarding areas, and finally 2E's main boarding area; as the bus circles around CDG2 clockwise, you have the longest possible airside transfer. You will not go through any immigration check, but you will need to clear security when entering the boarding area.