Brick rig shifted in kiln while rolling it to new position. Kiln stopped, rig was leveled and wheels chocked. Crew climbed up to adjust shifted pieces. While on rig, it rolled up kiln to the right (facing upkiln) and overturned. Employee's leg got caught and was injured, later found to be broken. Other 3 employees on the rig at the time of the incident were uninjured.