It's not just a matter of subtraction and substitution. It is worth pursuing if you live with serious dietary restrictions or if you like to experiment. They forget that the recipe is designed to produce many servings and the fat content per portion is low. Nutritionists now suggest the pendulum has swung too far in the 90's. They say many of us have an irrational fear of fat.
Some fat is required in the diet.
As with anything, moderation is the key. This low fat fetish has even reached into the realm of bread. Bread - unless it is a challah or a rich Easter bread is naturally low in fat. A 16 slice loaf of bread contains perhaps 2 tablespoons of fat - most of which is not even required. French and sourdough breads do not require any fat at all.