Hindi ko directly na-try yan sa premature formula, but a few of us mums talked about it, usually mas safe kung gradual ang transition, like you mix them for a week or two bago full switch, kundi baby's tummy might get upset. Since si baby mo premature, I'd really suggest asking your pedia first because the nutrients in premature formulas are specially made para sa kanya, at baka may reason bakit siya naka-premature formula pa, better safe than sorry talaga!