Your story is not that uncommon. Sometimes the disease presents itself as UC and then years, sometimes many years later, Crohn's presents itself. A small bowel follow-through will show whether Crohn's is visible in the small bowel. Your change of diagnosis does not mean that any of your GIs have been negligent or wrong, it means that the diesease hadn't manifested.
dx proctitis in 1987
dx UC in 1991, was stable until 1998
1998 started prednisone, asacol, pentasa, nortriptylene, ativan, 6MP, rowasa enemas and suppositories, hydrocortisone enemas, tried the SCD diet, being a vegetarian, omega 3s, flax, pranic healing, yoga, acupuncture, probiotics
2000 lost all my B-12 stores and became anemic
2001 opted for j-pouch surgery- now living life med-free