My cats eat different brands/flavours of food every day. I never feed them the same twice in a row and with all the fosters, I've never had a problem with changing their food too fast, too often.
I have found that lower quality food or kibble will affect their digestion more so than changing from a crappy to a quality one.
You may try Slippery Elm Bark (comes in capsules) to sooth the colon. Add a bit of water to their canned as it may inhibit the absorption of water.
I fed my kitties Nature's Variety Raw for years before I started making my own. It was a bit pricey for my budget. The only problem I found is that some older bags did have a bit of freezer burn on it.
Cat maid to:
Puddles, female Himalayan (16 yrs)
Jasper, male Ragdoll ?? (approx 9 yrs)
Sweet Pea, female silver tabby DSH (approx 7 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 6 yrs)
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey