1. The provisions of any decree respecting support may be modified only upon a showing of changed circumstances so substantial and continuing as to make the terms unreasonable. In a proceeding for modifications of any child support award, the court, 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 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.

2. When the party seeking modification has met the burden of proof set forth in subsection 1 of this section, the child support shall be determined in conformity with criteria set forth in supreme court rule 88.01.