Hi there,
I appreciate you reaching out in the Community. I'll be glad to share more information about editing invoices in QuickBooks.
Good job on customizing the Line Item Description. I see you have a good understanding of how QuickBooks works. The new description edit only applies to new invoices. You'd have to manually edit the pre-existing invoices if you want them to reflect the change. Let me know if you need help with that. Otherwise, feel free to ask other questions. I'll be happy to help. :)