43 Tex. Admin. Code § 217.51
When a truck is converted into a truck-tractor and the registration classification is changed from "truck" to "combination," an exchange of license plates is required; however, if a truck-tractor is converted into a truck and the registration classification is changed from "combination" to "truck" the license plates shall not be exchanged, unless the change involves a major permanent reconstruction change, such as when the frame of a truck-tractor is altered to accommodate the installation of a different type bed or body. In this instance, the owner must exchange license plates and file an application for corrected title. Under no circumstances will a refund in registration fees be authorized when a combination plate is exchanged for truck plates as the result of a reconstruction change.
43 Tex. Admin. Code § 217.51