I had to change an ADR from 11 to 7 for one of the nights on our upcoming trip and we were able to get same restaurant on same night but could not get the same time, had to go with a later time. However, that's still better than getting to the restaurant and having to wait for an opening for the smaller table size.
As a party of 11, we only have 3 reservations where we are not a party of 11 at one table; (example) if the CM did the reservation as a party of 6 and party of 5 at 2 tables, I was given this information when I booked my ADR's.