Colectomy doesn't cure you of the auto immune disorder that causes UC so it's a bit misleading to say the surgery is a cure. And this is something that should be communicated to the patient. Autoimmune disorder manifests as both UC and Crohn's in the bowels. And the course I have had of it manifesting as inflammation in different places isn't unusual. Once you remove the labels you are just talking about inflammation appearing in different places at different times- which isn't necessarily predictable to anyone.
I would have had a colectomy no matter what because at the time my entire colon was inflamed and none of my ileum had any inflammation at that time. The entire colon was severely inflamed and dissolving on removal and resection wasn't in the cards. That was after 20 years of UC that was not well treated. I just didn't respond well to the available treatments at the time. Surgery was inevitable, and when it ended up happening was semi-emergent.
15 years later Crohn's appeared in my ileum. What caused it was the same autoimmune disorder that caused the UC. Being strictured at the J pouch inlet doesn't help, but I am told my pattern is scattered ulcerations in the neoterminal ileum, which doesn't correspond neatly with backsplash stool- hence my Crohn's Diagnosis, because of the location and pattern of the inflammation.
I don't see how the latter diagnosis changes the fact that I was correctly diagnosed with UC 15 years earlier, and I am suggesting that this happens in a lot of other cases besides my own. Had the same biological drugs existed when I had my colectomy and they worked as well on the UC as they do on my Crohn's now, I wouldn't have needed a colectomy. But they didn't exist at the time and how they would have worked on inflammation of colonic tissue - which is different than the tissue of the ileum- is just speculation.