To handle a product swap/exchange, you can follow the steps outlined below.
Create a Return Authorisation and a Credit Note on the original Sales Order
Start by creating a return authorisation and a credit note from the existing order for the products that are being returned. This will ensure that the value of the returned products is credited back to the customer.
Click on [+ New Order]
Create a new sales order for the products that the customer wants to receive in exchange. You can use the order tag Swap to easily identify this transaction as a swap.
You can create order tags in your System Settings.
Read Sales Documents Settings - SO Tags to learn more.
You can use the Comments section to note the original sales order.
Finally, when you are ready to ship, create a delivery note and an invoice on the new sales order and mark the credit note on the original sales order as applied.