When the proceedings are reopened to increase, decrease or terminate maintenance and support for a spouse or for a child, or to request additional orders to effectuate previous orders entered pursuant to § 20-107.3, the petitioning party shall give such notice to the other party by service of process or by order of publication as is required by law. Except as provided by § 20-110, no support order may be retroactively modified, but may be modified with respect to any period during which there is a pending petition for modification in any court, but only from the date that notice of such petition has been given to the responding party.