@computersci@txborn DS asked this question of an admissions rep at the Best of Texas event last weekend. She told him to fill out the Appeal form on the admissions dashboard.
It might seem a little strange because it will ask you if you made a mistake on your application but, that is the correct form. Apparently, they will allow you to potentially switch majors one time before orientation. The original admissions application will be reviewed by the new requested major and they will get back to you.
This would be only for accepted students who have accepted their offer and have not started classes yet. After you get on campus, the process is completely different. There is also no guarantee that the major change will be approved or that the new major will have space.
DS did ask for a change within the school he was accepted to so, we’ll see if he gets it.