I reduced lactose almost immediately upon realizing I was lactose intolerant.
I tried adding fiber (most UC patients get this advice) but I felt much worse and so then went the other direction and ate only low residue foods for a while. Neither approach really helped me. When I'm flaring the best thing is to just not eat.
I did reduce the quantity of raw vegetables in my diet. That kind of happened naturally. I ran out of interest in preparing food, I ran out of energy to eat a lot of salad, thus the end of salads at every meal. Now I eat a salad about
5x/week.
I added in more lean protein. I was eating a primarily vegetarian diet, but legumes turn out to be pretty hard on my gut so I cut way back. Plus there's the whole lactose intolerance thing and cheese....and I felt better in general when I had more lean protein. I also have been chronically anemic, so I needed more red meat. Now I have meat about
2x/week, whereas before maybe 2x/month.
I eliminated gluten completely this summer. Very significant improvement in my mood, energy level and general belly happiness.
ADDED: since you asked about dietary changes, I did not mention the fact that I have not eated artificial sweeteners, or much processed food of any kind, for a long time; that all stopped well before I ever had an IBD diagnosis.