This is so important! I'm glad you're catching this now, @Ty3535!
If I had done something and taken it more seriously back when she had started vomiting here and there, for seemingly no reason, she would likely have not gotten the thickened intestinal wall and so much inflammation.
We knew absolutely nothing about IBD when our cat started having sporadic symptoms--also when she was around eight--and, unfortunately, her vets didn't know much more about it than we did and she gradually got worse over the years. (She lived a pretty long life, she was at least 17, despite everything!) If a vet brushes off the symptoms (anything from hairballs to overgrooming), find another vet, preferably a cat specialist.
It was really helpful for me to keep a diary of Brooksie's food, vomiting, and litter box activity. Pretty much anything can cause IBD but for her it seemed to be grains and fish.It's been a journey to me at least, trying this and that but first figure out what all is in your cats curent food now.
Fish we know, is it really just about all fish?
If so that will be much easier to work out.
Is there an ingredient in the new food that your cat hadn't eaten before?
Good luck!