Colic and feeding- how often can you switch formula brands?

my DD never tolerated formula its based on cows milk and alot of babies have trouble try the gentle ease thats more broken down if not try the hypoallergenic stuff noutrimagen or alimentum... although u can never get away from the fact that BM is just easier on the belly ive been EPing since my DD was 7 months simply bc she started refusing the breast but never tolerated any formula