(a) If a retiree who is less than 60 years old and who is receiving a disability retirement annuity resumes employment with a participating municipality or otherwise becomes gainfully employed, the person automatically resumes membership in the retirement system, and the board of trustees shall terminate the person's annuity payments.
(b) If a person resumes membership under this section, the retirement system shall restore to effect any prior service credit, special prior service credit, antecedent service credit, or updated service credit used in determining the amount of the person's annuity at the time of disability retirement. If the person subsequently retires, the retirement system shall allow the person credit for all current service for which required contributions have been made and not withdrawn.