First of all does your child using the same brand of the milk power but just move on to next step? If YES then you can change it bit by bit, sample: baby drinking 8ml of milk then u should use 1ml for the next step so 7ml is the original. Next meal u do the same thing but 2 - 6ml then again 3 - 5 ....etc
You can do it slowly and that take sometime but again baby will not getting any diarrea. Hope this help u out.