G.B.
Just transition slowly, start with 2oz. whole milk to 6 oz. formula. If there are issues go back to whole formula then try again in a few weeks. If still issues then the child may need to consider other options for extending the formula for 12-18 month old toddlers or another milk substitute.
If they do fine on the 2 to 6 ration then after a week go to half milk and half formula. If that goes okay then go to 6oz. milk to 2 oz. formula. If that is good you can go ahead after a couple of days then go whole milk.
This way gives the child's body the chance to tell you how they are going to adapt to milk. It may take longer than a month to transition and it may just take a couple of weeks but the slower you go the better you can tell and the better the little ones body will adapt.