As Ellie F and MOMOFTHREE wrote, introducing cow's milk too early is associated with an increase in all kinds of health problems (allergies, asthma, eczema, ear infections, obesity, diabetes etc etc). *BUT* as Leanne B said, most of us survived quite handily despite being born in the dark ages when our parents pretty much gave us cow's milk from the get go :-)
The thing is, it's only a minority of people who are adversely affected by having milk too early. 90 out of 100 babies will do just fine, and grow up to be healthy adults. The other 10, however, are going to have problems. The question is, how do you tell if your child is likely to be in that 10%? I think a good thing to go by would be family medical history. If you or others in your family have any of the health problems associated with cow's milk, then you might want to stick with the formula. If your entire family is slim, fit and sparklingly healthy, then your baby is probably going to be fine with milk. If common sense tells you that milk-related issues don't run in your family, then give the milk a try. If she does start getting rashes and ear infections or whatever, then you can always go back to formula.
Don't ignore the medical evidence, just interpret it in the context of your family's individual health and experiences. Good luck!