On the date that any increase in the rate of tax imposed under this chapter takes effect, an inventory tax is imposed upon all distillates that are held in inventory by a supplier or retail dealer as of the end of the day prior to that date on which the tax imposed by section 3203 has been paid. The inventory tax is computed by multiplying the number of gallons of tax-paid fuel held in inventory by the difference between the tax rate already paid and the new tax rate. Suppliers and retail dealers that hold such tax-paid inventory shall make payment of the inventory tax on or before the 15th day of the next calendar month, accompanied by a form prescribed and furnished by the State Tax Assessor. In the event of a decrease in the tax rate, the supplier or retail dealer is entitled to a refund or credit, which must be claimed on a form designed and furnished by the assessor. [2009, c. 413, Pt. W, §3 (AMD); 2009, c. 413, Pt. W, §6 (AFF).]