I myself did not have any change in the schedule for my donation surgery (11 years ago), but it certainly CAN happen, so it is wise to have back-up plans ready. Both the donor and the recipient must be in sufficient health at the time of the surgery, so even a temporary problems (e.g. flu) can cause a postponement. And if it is a simultaneus multiple transplant a problem with any of the parts of the "chain" might cause a delay. It is also possible that an emergency situation can arise suddenly (e.g. multiple victim car crash) causing all postponable surgeries in a particular hospital to be cancelled beacuse all the operating room are filled. I don't really know just how frequent postponements of donation surgery are, but it is prudent to keep the possibility in mind.
best wishes,
Fr. Pat