Yes, I always dread this has happened when we do well on a food and then have problems.
After a couple of fecal tests to rule out bugs and some research (cross referencing for their kindey concerns) I think we will try the new Nutro Grain Free either venison or turkey. Although Dr. C. says the renal diet debate is split down the middle as to whether it makes much difference. Blue has some good ones also but they have peas which you avoid for kidney issues apparently.
My vet offered the Royal Canin also and to try to get us a Kangaroo blend but We'll try this first. It's not allergies, just sensitive tummies.
We've got some Met. and Fortiflora also to get things settled down.