Hi rowland ,
Apologies for any inconvenience caused.
When does the metadata get lost when you add fields to Purchase Orders? Added custom fields shouldn't disappear.
Anyway, you have the Customer's Price and the Sales Order Number fields to keep track of the original order and customer.
Indeed, when you select the order ID, you will be asked to confirm if you want to add the items from the Sales Order automatically, and the customer's name will also be imported.
Best Regards,
José Andrés