I tried to eliminate a day from a "free dining" bounce back offer. It was for the last night a Saturday. Cast member explained that since the promo was no longer available to me, I could cancel that night. They then wanted to add $1,600 to my package for the dining package for our group.
Adding people to the reservation, would be the same. If promo is no longer available, no.
I would definitely call, but don't cancel what you have. Just ask questions, about your reservation. It's Disney, but I did not have any luck.