If you update the custom field name for a product in the product details section in Swipe, the new custom field name will be reflected in the old/new invoices generated and the data added in the custom field on the previously created documents will not get affected.
Note: If the custom field does not appear on your invoice, Please check if you have entered any value in that custom field.
The custom field will only appear on your invoice if a value has been entered for at least one product in that field.