my 3 week old daughter is having tummy aches alot now we believe from the formula shes eating…

my 3 week old daughter is having tummy aches alot now we believe from the formula shes eating similac sensitive..we want to put her on enfamil gentlease because we heard it was more sensitive for tummies, we arent sure how we go about switching her? just quit the old one cold turkey? or is there a formula winging process? HELP!!