The retirement system and any related trust agreement, investment advisory agreement, custodial agreement, or similar agreement, may be amended by the county at any time, either prospectively or retroactively, to conform to the provisions of the Internal Revenue Code or similar act or amendment thereto or regulations promulgated thereunder. However, if the Internal Revenue Service requires an amendment and the amendment reduces a benefit provided by the retirement system, the required reduction must be paid as a separate benefit from a nonqualified supplemental plan to be established by the county. The benefit payable to a member from the retirement system and the related benefit payable from the supplemental plan, when taken together, must equal the benefit promised under the retirement system immediately before the amendment required by the Internal Revenue Service. (1987 L.M.C., ch. 44, § 5; 1988 L.M.C., ch. 25, § 1.)