(a) Any person who becomes a judge during the period of January 1, 1980, through December 31, 1988, shall not be eligible to be retired for disability unless the judge is credited with at least two years of judicial service or unless the disability is a result of injury or disease arising out of and in the course of judicial service.
(b) Any person who becomes a judge on or after January 1, 1989, shall not be eligible to be retired for disability unless the judge is credited with at least four years of judicial service or unless the disability is a result of injury or disease arising out of and in the course of judicial service.