(a) Unless both parties and the child reside in this state, a court of this state may modify an order of child support rendered by an appropriate tribunal of another state only as provided by Chapter 159.
(b) If both parties and the child reside in this state, a court of this state may modify an order of child support rendered by an appropriate tribunal of another state after registration of the order as provided by Chapter 159.
Official text of the Texas Family Code as published by the Texas Legislative Council.