You can easily transfer a retainer from one project phase to another by using the Transfer Retainer option in Core. You can do so by clicking the Retainers screen from the side menu > Billing > Retainers. Then select the relevant project phases from the respective drop-downs. From the More menu, select Transfer Retainer and click Transfer. The retainer amount is transferred to another phase. To know more about retainer transfers, check Core Help for details.