Adrian,
With direct deliveries, you can alter the dates on both the sales order or purchase order line.
Sales order lines - You'll update the requested receipt date field and it will update the purchase order line automatically.
Purchase order lines - You'll update the confirmed receipt/ship date fields, the changes will also flow through to the sales order.
If you're having to account for two sets of dates, have you considered canceling the remainder on the parent line, then creating a new line that captures the respective quantities and dates?
Others may chime in with some more commentary on alternative courses.
