Hi NinjaFlip, 

Thank you for reaching out on this and welcome to our community!

The best time to add existing invoices to projects is before they’re paid. If you add a paid invoice to a project, the invoice will disconnect from the attached payment. This will also change the invoice’s status to unpaid. You’ll need to follow the steps to reconnect the payment to avoid accidentally charging customers twice.

We don’t recommend adding invoices linked to more than one other transaction, billable expense, or timesheet. If you add a linked invoice to a project, only the invoice is added. You’ll need to add the linked transactions to the project and reconnect them to the invoice.

You can add existing estimates to projects as long as you haven’t converted them to invoices.

For more information on projects in QuickBooks Online, please visit the link down below: 

Set up and create projects in QuickBooks Online

Thank you and please let us know if you require any additional assistance.