If the agreement governing the credit card plan so provides, the rate or rates of interest under the plan may vary in accordance with a schedule or formula, and/or upon the happening of any event or circumstance specified in the agreement, including, but not limited to, the failure of the cardholder to perform in accordance with the terms of the agreement. The interest rates, as so varied, may be made applicable to all or any part of the outstanding indebtedness under the plan on or after the effective date of the variation, including the indebtedness arising out of extensions of credit made prior to the variation in the interest rates.