Loose stool - change in food or Prednisolone?

As some of you may remember, my mom's cat had a sore on her right hind
leg.
It's now clearing up thanks to a shot of Depo Medrol, and 2 applications
a day of Tresaderm + 2.5 mg/day of Prednisolone (crunched into powdered
form and placed in about one fifth or less of a slice of Boars Head roast
beef. Wow, does she love that. Those four little pieces are gobbled up
in about 10 seconds. She comes back licking her chops but unfortunately
(for her), I won't give her more.

Here's the problem. After about a week of Tresaderm and Prednisolone,
the little thing has moved her bowels about 7x in two day. The stool was
progressively looser each time and now she has a touch of diarrhea.
I thought it might be the 2.5 mg/day of Prednisolone, but we also
switched her diet from Fancy Feast canned food and Purina Proplan Chicken
and Rice Hard Food and Purino Pro Plan Sensitive Skin and Stomach Formula
Hard Food with the Fancy Feast canned food.

While diarrhea is a possible side effect of Prednisolone, our vet said
it's rare, and since she didn't have stool problems with it last year, he
thinks it may be the Purina Pro Plan sensitive food formula. He thinks
the abupt switch may have caused the diarrhea.

He suggested to eliminate the wet canned food, go back to Purina Pro Plan
Chicken and Rice, and if I want (my suggestion) skip a day of
Prednisolone.
He also suggested that when her stool is back to normal, slowly wean her
onto the Purina Proplan Sensitive food formula (which I think is more of
a lamb and rice formulation).

Anybody have similar problems? I didn't realize that a change in hard
food (of the same brand name) could cause a digestive problem.