According to the Secretary of State’s office, election division, the bill that changed the law was HB 1, Chapter 1, 78th Legislature, Third Called Session, 2003. This was the session in which the mid-census congressional redistricting map was adopted. The runoff was changed to the second Tuesday in April. Here is a link to the legislation: http://texinfo.library.unt.edu/sessionlaws/78thsession/bills/78cs3/HB1.pdf My thanks to the Secretary of State’s office and staff.