Should the medical board report and certify to the board of trustees that such disability beneficiary is engaged in or is able to engage in a gainful occupation paying more than the difference between his retirement allowance and his average final compensation, and should the board of trustees concur in such report, then the amount of his pension shall be reduced to an amount which together with his annuity and the amount earnable by him shall equal the amount of his average final compensation. Should his earning capacity be later changed, the amount of his pension may be further modified, providing that the new pension shall not exceed the amount of the pension originally granted nor an amount which, when added to the amount earnable by the beneficiary together with his annuity, equals the amount of his average final compensation. A beneficiary restored to active service at a salary less than the average final compensation upon the basis of which he was retired at age fifty-five or greater shall not again become a member of the retirement system and shall have his retirement allowance suspended while in active service.