It should be fine, I take it he/she is old enough?
I would begin to mix the two together ie substitute around 2oz of formula for cows milk for a few days then gradually increase the amount until its just cows milk.
I wouldn't advise just swapping it completely, I know somone who did and it made their baby sick!
This is also a good time to wean baby off a bottle if you haven't already and use a feeder cup or similar.
Good luck!



