So, what you're saying is there is no easy way to move a BUNCH of invoices (did not come from estimates, were just entered in as an invoice) to under a Project after they've already been sent and paid? You have to do them one at a time? That's totally inefficient and a logistics nightmare for companies that process dozens of invoices a day, as mine does.

I, literally, have been tasked to go back to Jan. 1 of this year (almost 7 months ago) to move dozens and dozens of invoices and expenses under a Project because management wants to see actuals for a specific project (a P&L of sorts)....but you're telling me it's going to take weeks/months to accomplish this because QB Online doesn't have a "MOVE ALL SELECTED INVOICES TO A PROJECT" function? 

Would it be simpler to use the CONVERT SUB-CUSTOMERS TO PROJECTS function?