If you already have the complete area you still an area completed you stay with the area complete effects (show raids, achivments ) completed (shows raids and achievements) but you still can discover the single new area and get the 100% complete again.( i remmeber again. I remember when scarlet Scarlett Etzel was laucnhed i didnt had the cobra bastion launched I didn't have the Cobra Bastion area discovered but still reciving ankrahmun raids notifications, I still received Ankrahmun raids notifications and also i still had the achivment)still had the achievement.