When we changed from formula to milk I exchaged one cup of formula for 1 cup milk. Two weeks later I'd exchange another cup (=2 cups) forumla for milk.
We get (3) 8 oz cups of milk a day. However, according to ped and food pyramid they should only be getting 2 cups. My children are 4 and 2.
You are right that milk is not as nutritious as formula, but by 15 months your child should be eating enough solid food to make up the difference. I can tell you that when it come to kids nutrition/eating habits all the doctors and nutritionists tell you to base their eating on a weekly basis rather than the daily version that used for adults. Kids can be picky eaters making it a creative challenge for the parents to make sure they get all vitamins/mineral they need :)
Good Luck.
Also, with the moving from formula to milk - this was a smooth process for us (ie little fuss except the very first day and that just took a stern look and tone saying we are growing up, if you want milk this is the kind you get and the cup you get it in). If they didn't want the milk but seemed thirsty they were offered water.
~A.