I have a similar situation with our trip this year. It was possible that our family of four would have two additional family members joining us, but weren't sure at the time we needed to book our ADRs. I knew it would probably be easier to reduce the number on our ressies, but wasn't sure if it would require a cancellation and rebooking or not, thus risking losing our ADRs. I called to ask a few days prior to our 180 day window and asked the same question when calling to make our ressies. Both times I was given the same response...I was told that while it would require a cancellation and rebooking, I would not lose my reservation. They did say that since we were initially a party of 6 that they would check to see if we were assigned a table for 6 or two tables together. If we had been assigned a 6 seat table, they would check for availability for a table for 4 around our original ADR time so that it could open up a table for another party of 6. We would not be forced to take the new time, but it sounded perfectly fair to me as long as my ADR time didn't change by much. If no other openings were available, however, they said we could keep our original reservation (and table for 6) which would just be reduced to 4. If we were assigned two tables "pushed together", obviously that would simply open up another table for someone else. Clear as mud? Since CM information seems to vary a LOT sometimes, I can't guarantee the accuracy of the info, but I was told that twice on two different days by two different CMs. If that makes it any more reliable...who knows! I should know for sure, though, in the next day or two as I just found out our other two family members will not be coming with us...so we'll see what happens.