N.W.
I am not sure what others have done but whne I switched my son to soy as an infant I just switched it and never went back. It didn't bother him because he handled the soy so much better. I switched him back when he was older to milk based just to make sure he cuold handle it before we got to weaning to regular milk.
But, each baby is different with formula, even milk based ones. i had one breast fed baby at daycare that took Carnation Good Start while at daycare, BM at home, he did fine. I had another breast fed baby that mom couldn't keep up, he puked Good Start but kept down similac just fine. So it may be that it's just the brand of formula. I personally lie Good Start and have had 4 different babies that use it. I would probably call the doctor and find out about the rash, but some babies just tend to spit up formula. It can be so frustrating can't it?!