An independent school district that used a method of selecting the assessor or collector of the school district’s taxes for the 1994 tax year that was authorized by former Subchapter F, Chapter 23, as that subchapter existed on January 1, 1994, but that is not authorized by § 45.231 or by Subchapter B, Chapter 6, Tax Code, may continue to use that method of selection until the school district uses another method authorized by § 45.231 or by Subchapter B, Chapter 6, Tax Code, to determine how the assessment or collection is performed.