(1) The medical status of each member receiving a disability benefit will be reviewed annually by the board to determine whether the member continues to be disabled, unless:

(a) the board determines reviews are unnecessary and may be discontinued;

(b) the board determines more frequent reviews are warranted by the nature of the disability;

(c) the board converts the disability retirement benefit of a participant in the defined benefit retirement plan to a service retirement benefit; or

(d) a participant in the defined contribution retirement plan receiving a disability benefit reaches 60 years of age.