When I first started making my reservations for dining there were going to be 4 of us going. Plans have changed; now there's only 3. Do I have to cancel my reservations and make new ones?
We had a similar situation. I just called Disney Dining, and asked them to change the number of people on each of the ADRs we made. As long as we were going down and not up, the CM said that it was no problem.
My family also had the same change (going from 4 to 3) and didn't have any problems with the ADRs. We didn't call Disney Dining before hand (we didn't know if the fourth was joining or not until the last minute), but did tell the cast member at check-in that we'd be three instead of four and they were all fine with it. I think this is because a table for three is typically the same as a table four, so it doesn't change the table assignment.