With the addition of Cobra Bastion to Southern Darama, what happens to those who had already explored Southern Darama 100%?
For such people, Southern Darama is discovered in 82% so they lose the benefits of discovering in 100%
Are the discovered areas reset and need to be rediscovered?
No, you need discover only new area to complete all Southern Darama for 100%
As for players who discovered enough areas and got both the outfit and the achievement, do they keep them or do they lose them?
You keep your achievement and outfit/addon but you will have to rediscover Southern Darama one more time to get an first addon (if you don't have it yet) or second addon in the future