Our doctor told us to do it gradually, at 12-13 months old - by mixing whole milk with the formula as follows: Our baby did 3 bottles a day at 7oz each, and a 4-5 oz bottle in the evening before bedtime.
SO--The doc said just start mixing whole milk with the formula as follows: Week 1-- 1 oz milk & the rest formula.
Week 2-- 2 oz milk & the rest formula, Week 3 - 50/50 milk & formula. Week 4 -- 1 oz formula, the rest milk. Week 5--ALL MILK. Our son seemed to notice...but never really made a big fuss, and continued to drink his bottles.
After week 5 - we started removing bottles, and replacing with a flexible straw cup. First the morning bottle, then a couple days later, both morning & lunchtime bottle. Then a couple days later, and all mealtime bottles were straw cups. He never seemed to balk. He likes being a "big boy" and using straw cups.
We still have the night time bottle 2 months later -- 4 oz of warm milk...because he had surgery and our doctor told us to put off removing this one a little longer, for emotional reasons...but he's completely well now...and we're about to start removing it. I've heard to start mixing water with the milk, little by little until it's just a bottle of water...then switch it to a sippy cup of water, and a cracker as a before bed snack, then brush their teeth...and do the bedtime routine as normal (stories, cuddling, songs) and they should get used to it.
Hope this helps!
A.