Tex. Fin. Code § 348.214

If a premium is added to the unpaid balance of a retail installment contract under Section 348.207 or 348.213, 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 348.113-348.117 without reclassifying the motor vehicle by its year model at the time of the amendment.