If the agreement governing the revolving credit plan so provides, the periodic percentage rate or rates of interest under such plan may vary in accordance with a schedule or formula. Such periodic percentage rate or rates may vary from time to time as the rate determined in accordance with such schedule or formula varies and such periodic percentage rate or rates, as so varied, may be made applicable to all or any part of outstanding unpaid indebtedness under the plan on or after the first day of the billing cycle that contains the effective date of such variation including any such indebtedness arising out of purchases made from a participating merchant or loans obtained prior to such variation in the periodic percentage rate or rates. Without limitation, a permissible schedule or formula hereunder may include provision in the agreement governing the plan for a change in the periodic percentage rate or rates of interest applicable to all or any part of outstanding unpaid indebtedness, whether by variation of the then applicable periodic percentage rate or rates of interest, variation of an index or margin or otherwise, contingent upon the happening of any event or circumstance specified in the plan, which event or circumstance may include the failure of the borrower to perform in accordance with the terms of the plan. Nothing herein precludes a licensee from charging or reserving a right to charge, by discretion or otherwise, a rate lower than any maximum rate provided for in any schedule or formula.