the exact location for changing your number plate is a bit tricky. It's not a police station neither a traffic office, it is only for changing plates. Take the d-ring road also known as Feb 22nd street or North Road. After landmark keep on going straight and when you pass the next interchange (gharafa interchange) look on your right hand side, you will see Qatar Petrol Station and then the big wall of the duhail army camp. Take the first right after the Petrol Station, it becomes kinda u-turn, drive back towards the petrol station on the service road, just before reaching the petrol station a small roundabout appears, take left from this roundabout and drive alongside the wall on your left hand, from next roundabout take a u-turn and now the army camp wall is on your right hand, now take the first right, yes inside the boundary of the army camp you will see the number plates changing office on the left hand. I know it is pretty long and well I don't know who was the genius that put it there.
cheers!