Notwithstanding the other provisions of this Article, the widow’s annuity payable to the widow of a fireman who dies on or after July 1, 1997 while an active fireman with at least 10 years of creditable service shall be no less than 50% of the retirement annuity that the deceased fireman would have been eligible to receive if he had attained age 50 and 20 years of service on the day before his death and retired on that day. In the case of a widow’s annuity that is payable on the effective date of this amendatory Act of the 93rd General Assembly, the increase provided by this Section, if any, shall begin to accrue on the first annuity payment date following that effective date.