For the new year, I've switched from 2% milk to 1% for my morning cereal (of course I'll still cook with whole milk or half and half, as the situation requires). Some calculations:
Apparently a serving of 2% has 4.5 g fat and 120 calories, while a serving of 1% has 2.5 g fat and 100 calories. There are 8 servings in a carton, and I go through about 1 carton every two weeks (I eat cereal every day for breakfast. I know my physique may lead casual observers to believe that I eat bacon omelettes, but sadly grape nuts are much more common). That means I consume approximately 208 servings of milk a year. By that estimate, switching to 1% means that if this was the ONLY change I made to my diet, I would save 516 g fat and 4160 calories this year.
As I say in the title though, I'm about 100% sure it won't make any difference. I'm sure I'll make the calories and fat up in some other, inscrutable way.