Eating In: Dr. Smiths

Switching Formula....a Strange rash...and Baby Dandruff???

I have 2 questions. The first pertains to switching formula. I previously asked about brands and types of formulas to add into my 5 mo old's breastfed diet. I was giving him soy because I thought it might be he easiest to digest and based on all the responses I received, I've decided to switch to Good Start. Am I supposed to switch it gradually(like when switching dog foods??) or can I start the new formula right away? How will I know if he's allergic to dairy? I only give him 2-3 ounces every 24 hrs in addition to his breastmilk. I do this...