CF is CF. It is progressive. When a child is born it will most likely start with mild CF and it will progress. For some it will progress slowly, for others more rapidly. The only real way of describing CF with mild/moderate/severe is the amount of lung disease. When the lung function is above a certain place it is said to be mild. As it drops it goes to moderate and eventually severe and end stage.
Otherwise, as frustrating as it is to hear, CF is CF and at this point there is very little to predict it.