this is what i did for my kids when going from formula to cows milk (DD 4 & DS 17 months)

100% formula
3/4 formula - 1/4 cows milk for a couple of days then...
1/2 formula - 1/2 cows milk for a couple of days then...
1/4 formula - 3/4 cows milk for a couple of days then...
100% cows milk

this worked really well for my kids, but i did find my kids did not like and also vomited on full-cream cows milk, so i after speaking with the maternal and child health nurse i gave my kids REV (NOT skinny milk). we have had no ill effects from REV.

good luck!