In Harrison v. Cox, 524 S.W.2d 387 (Tex. Civ. App.--Fort Worth 1975, writ ref'd n.r.e.), the new law did not change the obligee's substantive right or the obligor's statutory duty, but merely provided an additional "means of enforcing that liability of a parent to support his child." Harrison, 524 S.W.2d at 392.
"It is settled that laws which affect only the remedy or procedure are not within the scope of the constitutional prohibition against retroactive law, unless the remedy is entirely taken away or is so encumbered with conditions as to render it useless." Id. at 391.
In Harrison, the court held that the new law could be applied to support payments due before its effective date. Id. at 392.