Here's the story - 8 year old neutered male - was starting to eat less
and less - had been on Purina One for years (with a treat of canned
food ev. 4th day)  but suddenly not liking it - switched to Iams
chicken - he gobbled it down - a few days later, he has a diarr.
problem, and even bloody stools - took to vet - cat was down a pound in
weight - vet said I'd probably switched him too fast - cat had a form
of colitis -  gave him pills for 3 days, bland liquids for a few days,
then introduced the Iams again verrrrry gradually -
now, 10 days later, he is fine, regular stools,  eating about 3/4 C
Iams a day, drinking 1/2 c water daily.

Do you think it was the rapid change of food, the fact that it was to
Iams, or something else coincidental?

Should I try to make any other dietary changes?

Thanks.