Notwithstanding the provisions of § 15-5-16.2, the court, in its discretion, may modify a child support order retroactively only to the date that notice of a petition to modify was given to the adverse party if it finds that a substantial change in circumstances has occurred. The court shall set forth in its decision the specific findings of fact which show a substantial change in circumstances and upon which findings of fact the court has decided to make its order of modification retroactive. The court, in its discretion, may for good cause shown suspend payment of child support arrearages until there is a finding by the court of financial ability to make payment on arrearages; provided, that incarceration for nonpayment of child support shall not constitute good cause to suspend payment of child support or child support arrearages.