Changing from nutramigen to regular formula?

my daughter is 7 and a half months and she has been on nutramigen since 3 months and does great on it but wen i went to her pediatrician last week he told us by next month we have to start her back to regular formula , he sai that if we dont change it we will have problems later when its time to switch her to regular milk.Iknow she will be in so much pain cause last week i ran out of nutramigen and i ve her getlease enfamil and she was in soo much pain.My question is did anyone else change from nutramigen to regular formula and do good? and if so how long did it take them to do better? or if any other suggestions please im very concerned..thanks