C.C.
According to my little sister, Consumer Reports did a study on baby formula a while back. So might want to try the consumer reports website and see if you can get any info from it without a membership.
As I remember from her the Sam's Club and Costco brands for both Similac and Enfamil are top notch. So I wouldn't worry about switching.
To make it easier on the tummy even though it is really very similar (if not the same! because sometimes the generic stuff is) you could make the bottles half and half for a a week, the 3/4 (new) to 1/4 (old) for a week, then full on new stuff the third week. Sometimes doing it that way makes it a little easier.
Good Luck!