The owner/operator of a skid-steer front-end loader was attempting to change a loader attachment. He exited the operator's seat, raising the safety bar to the non-operating position. He then lowered the safety bar to the operating position without returning to the operator's seat and the protection of the safety bar. He was trying to raise the boom to position the loader attachment by standing on the front frame of the loader and manipulating the controls with his left foot. His foot activated the boom, causing it to suddenly swing up, fatally crushing him between the boom and the cage support.