Cal. Veh. Code § 9406

Alterations or additions to registered vehicles for which fees have been paid under Section 9400 or 9400.1 placing the vehicles in weight fee classifications under Section 9400 or 9400.1 greater than the weight fees previously paid shall be reported to the department and at the same time the difference between the weight fee previously paid, reduced as provided in Section 9407, and the greater weight fee, reduced as provided in Section 9407, shall be paid to the department upon the operation of the vehicles in the greater weight fee classification under Section 9400 or 9400.1.