It's true that disc degeneration can occur at the levels above and below the first area fused. The result can be disc pressure on the spinal cord in the neck or on the spinal nerve roots as they leave the cord.
Many studies have been done on this topic. The results vary from as little as seven percent to as high as 50 precent years later. A recent study from Japan has shed some light on this problem. It seems that breakdown is more likely when damage is already present at the disc level above or below the level fused.
It appears the problems later are just a natural progression of the same disease. The disc degeneration is already present but the patient doesn't have any symptoms yet. Many patients get better with physical therapy and drugs. A second surgery is only needed about six percent of the time.