You can try calling the dining line to see if they can help you, but they'll be doing mostly the same thing (looking for a new reservation for the correct party size). I would hop online and search for the ADR for the maximum party size you might have. Make sure you have the new ADR Before you cancel the old one (i.e., don't cancel and then search).
Now you're going from an odd to an even, so in most cases you would be at table for 10 anyway, but there are some restaurants that have seating for odd numbered parties. Many folks will say to just show up with the extra person, but there is always the chance that the restaurant is at capacity and the extra person will be turned away or you'll be treated as a walk-up. So it's better to have an ADR for the right party size.