Tex. Fin. Code § 353.211

If a premium is added to the unpaid balance of a retail installment contract under Section 353.206 or 353.210, the rate applicable to the time price differential agreed to in the retail installment contract remains in effect and shall be applied to the new unpaid balance, or the contract may be rescheduled in accordance with Sections 353.114 and 353.115, without reclassifying the commercial vehicle by its year model at the time of the amendment.