G.N.
My son was drinking breastmilk and formula when we started introducing whole milk (at 12 months). It was little by little. I can't remember what the ratio was but basically as the breastmilk supply got smaller, the whole milk got larger. Then we started working on not feeding him as much formula, just whole milk. We didn't go straight to a cup though. We still fed him the bottle at that time.
To my knowledge you are supposed to wait until they're 12 months before feeding them cow's milk.
I'm glad we introduced the milk little by little because I remember he got really gassy at first.