1. At any time after the entry of an order pursuant to sections 454.470 and 454.475, the obligated parent, the division, or the person or agency having custody of the dependent child may file a motion for modification with the director. Such motion shall be in writing, shall set forth the reasons for modification, and shall state the address of the moving party. The motion shall be served by the moving party in the manner provided for in subsection 5 of section 454.465 upon the obligated parent or the party holding the support rights, as appropriate. In addition, if the support rights are held by the family support division on behalf of the state, a true copy of the motion shall be mailed by the moving party by certified mail to the person having custody of the dependent child at the last known address of that person. A hearing on the motion shall then be provided in the same manner, and determinations shall be based on considerations set out in section 454.475, unless the party served fails to respond within thirty days, in which case the director may enter an order by default. If the child for whom the order applies is no longer in the custody of a person receiving public assistance or receiving support enforcement services from the department, or a division thereof, pursuant to section 454.425, the director may certify the matter for hearing to the circuit court in which the order was filed pursuant to section 454.490 in lieu of holding a hearing pursuant to section 454.475. If the director certifies the matter for hearing to the circuit court, service of the motion to modify shall be had in accordance with the provisions of subsection 5 of section 452.370. If the director does not certify the matter for hearing to the circuit court, service of the motion to modify shall be considered complete upon personal service, or on the date of mailing, if sent by certified mail. For the purpose of 42 U.S.C. § 666(a)(9)(C), the director shall be considered the appropriate agent to receive the notice of the motion to modify for the obligee or the obligor, but only in those instances in which the matter is not certified to circuit court for hearing, and only when service of the motion is attempted on the obligee or obligor by certified mail.

2. A motion for modification made pursuant to this section shall not stay the director from enforcing and collecting upon the existing order pending the modification proceeding unless so ordered by the court.

3. Only payments accruing subsequent to the service of the motion for modification upon all named parties to the motion may be modified. Modification may be granted only upon a showing of a change of circumstances so substantial and continuing as to make the terms unreasonable. In a proceeding for modification of any child support award, the director, in determining whether or not a substantial change in circumstances has occurred, shall consider all financial resources of both parties, including the extent to which the reasonable expenses of either party are, or should be, shared by a spouse or other person with whom he or she cohabits, and the earning capacity of a party who is not employed. If the application of the guidelines and criteria set forth in supreme court rule 88.01 to the financial circumstances of the parties would result in a change of child support from the existing amount by twenty percent or more, then a prima facie showing has been made of a change of circumstances so substantial and continuing as to make the present terms unreasonable.

4. If the division has entered an order under section 454.470 or 454.500, and an additional child or children not the subject of the order are born to the parties, the division may, following the filing of a motion to modify, service of process, and opportunity for a hearing pursuant to this section, modify the underlying child support order to include a single child support obligation for all children of the parties in conformity with the criteria set forth in supreme court rule 88.01.

5. The circuit court may, upon such terms as may be just, relieve a parent from an administrative order entered against that parent because of mistake, inadvertence, surprise, or excusable neglect.

6. No order entered pursuant to section 454.476 shall be modifiable pursuant to this section, except that an order entered pursuant to section 454.476 shall be amended by the director to conform with any modification made by the court that entered the court order upon which the director based his or her order.

7. When the party seeking modifications has met the burden of proof set forth in subsection 3 of this section, then the child support shall be determined in conformity with the criteria set forth in supreme court rule 88.01.

8. The last four digits of the Social Security number of the parents shall be recorded on any order entered pursuant to this section. The full Social Security number of each party and each child shall be retained in the manner required by section 509.520.