A police officer age 50 or older who is receiving a disability pension may by written application to the board, elect the disability pension option if the period during which a disability pension was paid when added to the period of active service equals at least 20 years. The election shall permit the officer to continue to receive a retirement pension for the remainder of his or her life of 1/2 of the salary at the date of the retirement on disability in lieu of any amounts which would have been payable to the officer under Section 3-111.