J.
L.,
When I switched my now 3 year old daughter, I started doing it in her bottle then moved to a cup full time. I had a hard time because she had it in her little head that milk came in a bottle, juice and water in a cup. It took me a day of caving in, then I tossed the bottles. She would not drink much milk for 2 days, but then took it like a champ. I freaked a bit because she was not taking it, but realized that she was still drinking water and having wet diapers and was fine.
My only suggestion is to mix the formula with the milk in smaller and smaller increments(an oz or 2) until it is nothing but milk. This is just so her little tummy has time to adjust to the change.
Good Luck!