GROUNDS FOR MODIFICATION OF CHILD SUPPORT.

(a) Except as provided by Subsection (a-1), (a-2), or (b), the court may modify an order that provides for the support of a child, including an order for health care coverage under Section 154.182 or an order for dental care coverage under Section 154.1825, if:

(1) the circumstances of the child or a person affected by the order have materially and substantially changed since the earlier of:

(A) the date of the order's rendition; or

(B) the date of the signing of a mediated or collaborative law settlement agreement on which the order is based; or

(2) it has been three years since the order was rendered or last modified and the monthly amount of the child support award under the order differs by either 20 percent or $100 from the amount that would be awarded in accordance with the child support guidelines.

(a-1) If the parties agree to an order under which the amount of child support differs from the amount that would be awarded in accordance with the child support guidelines, the court may modify the order only if the circumstances of the child or a person affected by the order have materially and substantially changed since the date of the order's rendition.

(a-2) A court or administrative order for child support in a Title IV-D case may be modified at any time, and without a showing of material and substantial change in the circumstances of the child or a person affected by the order, to provide for medical support or dental support of the child if the order does not provide health care coverage as required under Section 154.182 or dental care coverage as required under Section 154.1825.

(b) Except as provided by Sections 231.1015, 231.1016, and 231.1017, a support order may be modified with regard to the amount of support ordered only as to obligations accruing after the earlier of:

(1) the date of service of citation; or

(2) an appearance in the suit to modify.

(c) An order of joint conservatorship, in and of itself, does not constitute grounds for modifying a support order.

(c-1) Incarceration of a child support obligor in a local, state, or federal jail or prison for a period exceeding 180 days is a material and substantial change of circumstances for the purposes of this section.

(d) Release of a child support obligor from incarceration is a material and substantial change in circumstances for purposes of this section if the obligor's child support obligation was abated, reduced, or suspended during the period of the obligor's incarceration.