J.,
I had samples of several different formulas when my daughter was born. Some were it travel packs, smaller containers, etc. The kind of formula I bought to use was Parent's Choice (Wal-Mart), or the Kroger brand. Her first few months, she had similac, enfamil, parent's choice, and one other kind. I didn't know you were supposed to stick with one kind. I just thought they were all pretty much the same thing. I didn't reasearch it much before she was born because I thought I would breastfeed, but my milk never came in. And I didn't think to look it up later, because she was fine. Anyway, she never had any problems at all with the switching, so I think you could probably just switch over and she will be fine, if she hasn't had tummy issues in the past. The only thing I would make sure is that if you have milk-based, switch to milk-based, soy-based to soy-based.
A.
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