I have switched formula so many times, I've lost count. I just had to switch from Target brand similar to Enfamil Gentlease to Good Start due to excessive spit up. I just switched cold turkey, never slowly. Lily never had a problem.
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Our pedi said to mix the formula 50/50 to move from Gentilese to Premium...but we are waiting for LO to hit 3 mos. first so I don't have experience...Good luck!
Re: switching formula...