If maintenance or
child support is modified by the court, the modification should be effective as of the date of the filing of the motion,
unless the court finds that it would cause undue hardship or substantial injustice or unless there has been a mutually agreed upon change of physical custody of the child.
In no instance shall the order be retroactively modified prior to the date of filing, unless there has been a mutually agreed upon change of physical custody.
The court may modify installments of maintenance or child support due between the filing of the motion and the entry of the order even if the circumstances justifying the modification no longer exist at the time the order is entered.