I was in the exact same situation before my mother-in-law commited to which days she was joining us. I had reservations for 6. I called and asked what I should do if the MIL joined us and was told that it depended on the types of tables they have at the restaurants. Some of my reservations were at tables that seated 5 or 6 and others were at tables that seated 6 or 7 (I'm guessing those are round tables????) In order to add a 7th person to the restaurants that we were booked at a 5/6 size table we had to cancel and rebook. For the tables that are 6/7 they just change the number on the reservation. The restaurants that I have been able to change from 6 to 7 and back without cancelling are Boma, Jiko, Princess Storybook, and Whispering Canyon. But I had to make all new reservations at Chef Mickeys, 1900 Park Fare, and Tuskerhouse. I was told I couldn't get a table for larger than 6 at Yak and Yeti or Sci Fi and we would have to get separate tables (which I declined.)