(a) Beginning January 1, 1999, the minimum retirement pension payable to a police officer with 20 or more years of creditable service, the minimum disability pension payable under Section 3-114.1, 3-114.2, 3-114.3, or 3-114.6, and the minimum surviving spouse's pension shall be $600 per month, without regard to whether the police officer was in service on or after the effective date of this amendatory Act of the 91st General Assembly. In the case of a pensioner whose pension began before the effective date of this amendatory Act and is subject to increase under this subsection (a), the pensioner shall be entitled to a lump sum payment of the amount of that increase accruing from January 1, 1999 (or the date the pension began, if later) to the effective date of this amendatory Act.