Sorry to sound negative, but I think you won't be able to get Applecare cover on your new display. To get Applecare on both Mac Pro + Display, they must be purchased together and appear on the same invoice. Apple seem to be quite picky about this, you'll probably end up having to purchase the seperate Applecare policy for the Cinema Display.
Saying that, I hope I'm wrong, but you'll not know until you ask Apple to clarify