(a) The court may consider the child support guidelines for single and multiple families under Chapter 154 to determine whether there has been a material or substantial change of circumstances under this chapter that warrants a modification of an existing child support order if the modification is in the best interest of the child.
(b) If the amount of support contained in the order does not substantially conform with the guidelines for single and multiple families under Chapter 154, the court may modify the order to substantially conform with the guidelines if the modification is in the best interest of the child. A court may consider other relevant evidence in addition to the factors listed in the guidelines.