I had to do the same thing on our last visit. I, too, was adding 1 person to a ADR for 3 so that may have been easier to do then your situation. But, I called Disney Dining as soon as I knew that I may need to do this. The CM looked at all of our ADR's to see at what type of table we would be seated and told me then it would be no problem. (so, it may be different at different resteraunts.) As soon as I knew that we would definitly need to add 1, I called back and added.