The way we do it in child care is like this.
The first week add 2 oz. of whole milk to the bottle, add 6 oz. of water, the formula powder and mix. That way the 8 oz. bottle is 1/4 milk and 3/4 formula. This may take more than a week. If they don't like it try a bit less milk but it should not make a real difference. Once they have successfully accepted the change and you are not seeing stinky poo or any other signs of their bodies not liking it go to half milk and half formula.
This one stage seems to me to be the one that is the longest. The formula/milk mix does taste different and they may not want it. I added a bit of strawberry syrup to the bottle or Hershey's syrup. I don't think it matter much. They will get used to it, it may take a day or it may take 2 weeks. Once they are doing it well and are used to it then go 1/4 formula and 3/4 milk.
That goes shorter than the other stages to me, once they take it a few times with they don't go back to the formula.
This is how I have successfully transitioned hundreds of kiddos.