I appreciate your time getting back here, Keely.

You will get this prompt when an invoice is converted from a billable expense. Changing the customer's name to a different project will not transfer invoice because it is already attached to the original project.

What we can do is delete the invoice and open the bill to update the customer's name from the Customer/Project column. 

To delete the invoice, here's how:

1. Go to Projects on the left panel.
2. Select the project name and click the Transactions tab.
3. Open the invoice, click More located at the bottom and select Delete.
4. Click Yes to confirm. 

Once done, go back to the Transactions tab and open the bill.

Here's how:

1. From the Transactions tab, click to open the Bill.
2. Change the project's name from the Customer/Project column.
3. Click Save and close.

Finally, open the project where you assigned the bill and click the Create invoice link on the billable expense. Here's a sample screenshot:

Feel free to read this article for more details: Projects FAQ.

If you have more questions about projects or other concerns with QuickBooks Online, please let me know in your reply. I'll be around to help you some more. Thanks.