I am thinking you really need to do this gradually!
I answered this about whole milk a while ago...But you can use it for formula too! Just replace 'whole milk' with 'formula' and adjust the age as necessary.
I started transitioning my daughter from breastmilk to whole milk at 11 months. I did it gradually, replacing a few ounces per meal and then waited 3 days before increasing it. For example, if taking a 8 ounce bottle per feeding, then what I did was this...7 ounces breastmilk/formula and 1 ounce whole milk per bottle. Waited 3 days to make sure everything was fine. Then 6 ounces breastmilk/formula and 2 ounce whole milk per bottle and waited 3 days again. Then 5 ounces breastmilk/formula and 3 ounce whole milk per bottle and waited 3 days again. And so on until you have completely transitioned to whole milk.
Good luck to you!