Yes, colon cancer can be caused by changes in bowel habits, including changes in shape and caliber.
A sigmoidoscopy and barium enema are adequate tests to screen for cancer, but as you suspected, a colonoscopy would be the definitive test.
If you a truly concerned about colon cancer, I would pursue the colonoscopy. There are some institutions that do a "no-sedation" colonoscopy, but you have to be a motivated patient.