Gina, going down a size requires you to take the jacket lining out, take in the seams at least 1/2 inch all the way around, taking in the collar, shortening the sleeves from the top, measure the shoulders once you take out the sleeves, the shoulder pad, and the sleeve head. You will almost be making a new jacket in order for it to look right. You will also have to take in the lining. You shouldn't have to do any alterations to the pockets but to the shell of the jacket, padding and lining, you will. Mary Roehr has a wonderful book on altering ready to wear clothing for men and one for women. I use them constantly as I alter clothing for my church members. Call me if you need help.
Marty
Brighten each corner where you are ... smile!