You will be able to search for a table for your party's size once their window opens. It will not necessarily be at the same time you have already reserved.
Typically I reserve what I can and then add to my reservation onboard on the first day. If I'm unable to figure it out before sailing I try to leave SOMEthing on the books. It seems easier to add then to create new. They are very accommodating and will do everything in their power to seat you together. This past cruise we went from a table for 4 to a table for 8 onboard (two of our four cabins were silver, rest were gold/platinum). We had to change our time by a half hour for dinner and choose another brunch day but we all got to eat together and it was no bother.