Our baby girls were born last week at 35 weeks 6 days and have been in the NICU the past week. We are hoping to bring them home within the next week or so. They are exclusively formula fed and have been on the hospital's formula, Gerber Goodstart. Once at home I was hoping to use Similac since we have quite a stash from friends who gave me their free samples and coupons. Is it difficult for them to switch formulas? I don't want to do it if it will cause more stress but I would like to use the formula we have if possible. I will have to change nipples on them anyway so I'm hoping the transition won't be too difficult. Anyone have experience with this?