Tex. Civil Practice and Remedies Code § 16.068: AMENDED AND SUPPLEMENTAL PLEADINGS.

If a filed pleading relates to a cause of action, cross action, counterclaim, or defense that is not subject to a plea of limitation when the pleading is filed, a subsequent amendment or supplement to the pleading that changes the facts or grounds of liability or defense is not subject to a plea of limitation unless the amendment or supplement is wholly based on a new, distinct, or different transaction or occurrence.

Acts 1985, 69th Leg., ch. 959, Sec. 1, eff. Sept. 1, 1985.