If I add the custom field for product without default value in the product details section, will that be reflected on my previous invoices?

If you add the custom field for product without default value in the product details section in Swipe, it will not get reflected in previously created invoices. The added custom field of product will be reflected in new invoices generated after the update only when you have added any value in that field.

Note: If you want to show that custom field in the previous invoices, you can manually edit the invoice and add the value in that field.