Va. Code § 20-108
The court may, from time to time after decreeing as provided in § 20-107.2, on petition of either of the parents, or on its own motion or upon petition of any probation officer or the Department of Social Services, which petition shall set forth the reasons for the relief sought, revise and alter such decree concerning the care, custody, and maintenance of the children and make a new decree concerning the same, as the circumstances of the parents and the benefit of the children may require. The intentional withholding of visitation of a child from the other parent without just cause may constitute a material change of circumstances justifying a change of custody in the discretion of the court.
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.
Any member of the United States Army, Navy, Air Force, Marine Corps, Coast Guard, National Guard, or any other reserve component thereof, who files a petition or is a party to a petition requesting the adjudication of the custody, visitation or support of a child based on a change of circumstances due to one of the parent's deployment, as that term is defined in § 20-124.7, shall be entitled to have such a petition expedited on the docket of the court.
Va. Code § 20-108