If the agreement governing a credit card plan so provides, the periodic percentage rate or rates of interest under the plan may vary in accordance with a schedule or formula. The periodic rate or rates may vary from time to time as the rate determined in accordance with the schedule or formula varies and the periodic 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 the variation, including any indebtedness arising out of purchases made or loans obtained prior to the variation in the periodic percentage rate or rates. Without limitation, a permissible schedule or formula pursuant to this section may include provisions 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, but not be limited to, the failure of the borrower to perform in accordance with the terms of the plan.