I have changed DD's formula before a few times and it never messed with her system. Mind you I wasnt swapping from gold ones to normal or soy or lactose intolerant, etc. Just the same type but different brand. I ended up sticking with the heinz one (cheap) and I think I read on here that they are all basic the same.

We are now on cows milk and no formula. We made the full change a week ago. I love it now. No mixing formula! If I am out i can go and buy a small milk and pour it ina cup or bottle for DD.

GL. I think every child and their digestive system is different but if DD isnt having any adverse reactions I would say its okay