The provisions of Article 58 of the new Act apply to the amount to be added to a refund or an overpayment or payment by mistake pertaining to national tax (including its expenses for the delinquent tax collection procedure) for which a payment decision is made or which is appropriated after the enforcement of this Act; provided, however, that the provisions then in force continue to govern the calculation of all or part of the amount to be added that corresponds to the period before the enforcement of this Act.