From what I am seeing in the DAC, the UnitWeight of the item is retrieved once, when adding a line on the SO. Afterwards, the calculations of the order weight are made based on the quantities and what has been already retrieved.
Without customization, the only way that I know to modify the weight of a SO would be to remove and add the line back.
However, if the weight is used to charge freight fees, you may want to change to weight of the package (editable box) so that the total weight of the SO will be good.
If you only want the weight to show on the invoices and other documents sent to the customer, you can calculate the weight based on the value of the item by changing the corresponding forms with the Report Designer.