M.P.
I have always started any change very slowly. I would suggest mixing the formula with the milk, maybe 3/4formula to 1/4 milk to begin with.(Maybe a little less if the baby has had any digestive track issues). I have stayed at that proportion for about a week and gradually increased the milk. This takes some time but I found that it really helped the transition not only for the taste difference but also without upsetting the digestive track.