We had to change our ressie number twice this trip. Once for Chef Mickey's we lowered it by two, because my younger son came down with a headache he just couldn't overcome by the time we had to leave, so he and DH stayed back. As my older son and I were on our way, we called the other family we were meeting and told them to let the CM know, which they did. We had 6:30 ADRs, and didn't get seated until 7:10 or so, but I can't say that the # change was the reason. (We were seated at a table for 6 rather than 8, though.)
Then, because of the younger one's unexpected night in, our evening plans for our last two nights changed. So, on our last night, DH and I ended up alone for our meal, while the boys played at Disney Quest. Again we showed up (Raglin Road) with two less, and had no problems at all.