Yes, coronary artery disease can be treated with lifestyle changes - exercise along with diet/stopping smoking/losing weight and reducing stress.
However in certain kinds of coronary disease (may need both a coronary angiogram and nuclear scan) where there is a severe blockage which is causing impairment of blood flow to a significant part of the heart muscle, it maybe advisable to proceed with heart stent or bypass surgery to prevent a heart attack with poor outcomes including mortality.