Once each year during the first five years following the retirement of a member on a disability retirement allowance and once in every three-year period thereafter, the board of trustees may require a disability beneficiary who has not yet attained retirement age as specified in subsection (a) of Code Section 47-2-110 to undergo a medical examination, such examination to be made at the disability beneficiary's place of residence or other place mutually agreed upon, by physicians designated by the medical board. The disability beneficiary may request such an examination. Should any disability beneficiary who has not yet attained retirement age refuse to submit to such medical examination, the pension of such disability beneficiary may be discontinued by the board of trustees until the withdrawal of such refusal; and should the refusal continue for one year, all rights of the disability beneficiary in and to a pension may be revoked by the board of trustees. Should the medical board report and certify to the board of trustees that a disability beneficiary is engaged in or is able to engage in a gainful occupation paying more than the difference between the disability beneficiary's retirement allowance and the earnable compensation the disability beneficiary was receiving at the time of retirement, the board of trustees may reduce the disability beneficiary's pension to an amount which, together with the disability beneficiary's annuity and the amount earnable by the disability beneficiary, equals the earnable compensation the disability beneficiary was receiving at the time of retirement. Should the disability beneficiary's earning capacity be later changed, the amount of the pension may be further modified, provided that the modified pension shall not exceed an amount which, together with the disability beneficiary's annuity and the amount earnable by the disability beneficiary, equals the earnable compensation the disability beneficiary was receiving at the time of retirement.