Adding to the cry for a fix on this. It is a huge issue for QBO customers and renders Projects a weak feature… when it could be an extremely powerful one.

Keeping linked transactions and attachments if customer/project is updated on an invoice is straightforward logic that should have been incorporated from the beginning, imo, and it has been a very long wait for the correction. 

We need the ability to bring older invoicing into our Projects reporting and to correct the original invoicing "customer" field when needed, even after adopting Projects — mistakes can happen using Project's sub-customer design and should not be so difficult to correct. As a small business, we outsource our regular bookkeeping and I've got a lot of unusable Projects reports b/c the bookkeeper was not consistent with the customer field. I should be able to EASILY and WITHOUT LOSING TRANSACTIONS OR HISTORIC RECORDS go in and update this field to pull proper Project reporting. I can't.