There's a whole thread of people over on the dining plan sub-forum discussing this, but to answer quickly.....
If you want to be sure you get it early, you'll want to go online sometime between midnight and 7:00 a.m. tonight/tomorrow and modify your existing reservation or book a completely new reservation. No one knows when it will be available to book online.
OR you can wait and call in at 7:00 a.m. and be on the phone for hours.