Since you are going from 7 to 8, there shouldn't be a problem. If you went from 8 to 9, there could be. Disney "reserves" things by table size. Whatever combination you have right now will more than likely have one extra seat.
Call and see what they can do. If you show up with one more and don't tell them, you could have to wait as a stand by. You could also try and if you can't do one, stop by that restaurant on the day of the meal and talk to someone. When you call the main number, you aren't talking to someone that knows exactly that is going on at that restaurant on that day.