Hi! I just changed our party size from 6-7 as well. I had booked my ADR's at the 180 day mark anticipating FD...
Sadly, you can't just add 1, as a PP stated you have to make new ADR's. I was able to get about half the ADR's I had planned revised.
I did some creative work (my extra people are my adult sisters and can dine separately from us-so this may not work for you).
Garden Grill, I couldn't add 1 so I made an ADR at San Angel Inn for my sisters at the same time.
Le Cellier-made them an ADR at Les Chefs de France for same time.
'Ohana-made them an ADR for Kona Cafe at the same time.
Chef Mickey's-I couldn't get a reservation for 7 people. But I was able to keep the ADR I had and make a separate ADR for 2 people for the same time as my other ADR-so we will all be in the same place at the same time.
These were the one's I was able to re-book for 7 people:
Coral Reef
1900 Park Fare
Boatwright's
T-Rex
Donald's Breakfast
Liberty Tree Tavern
For us, I thought this may actually be pretty nice-we will be in connecting rooms in the resort and all together most of the day, so it may provide some "break time" for all of us.
Good luck with your ADR's!