Veh. Code, § 9408

(a) Whenever any registered commercial vehicle, including, but not limited to, any commercial vehicle operating in California with apportioned registration, for which fees have been paid under Section 9400 or 9400.1 is withdrawn from service in this state before the expiration of the registration, the owner may surrender the registration card and license plates previously issued for the vehicle to the department and, within 90 days of the time of withdrawal, make application for the registration of another commercial vehicle which is subject to the fees specified in Section 9400 or 9400.1. If the vehicle that is withdrawn from service is operating in this state under Article 4 (commencing with Section 8050 of Chapter 4, credit for any unused fees paid under Section 9400 or 9400.1 may be applied only to a commercial vehicle concurrently added to the same apportioned fleet.

(b) Under the circumstances described in subdivision (a), and upon a proper showing of the facts, the department upon determining the fees payable under this division shall allow as credit thereon the unexpired portion, as of the month of the application, of the fee paid under Section 9400 or 9400.1 for the previous registration, but, in addition to fees otherwise payable under this division less any credit, shall charge and collect an additional fee of two dollars ($2) for issuance of the new registration.