Hi, @teambrown. 

Converting an invoice into a purchase order (PO) and vice versa is not possible in QuickBooks Online (QBO) since POs are for vendors and invoices are for customers. As a workaround, you can copy the purchase order into a bill and make it billable so it can be added to the invoice.

Here's how:
 

1. Create a Purchase order by clicking the +New button.
2. Once done, open the transaction and click the Copy to Bill tab.
   
   
    
3. In the bill window, check the box under the Billable column to make it billable.
   
   
    
4. Click Save and close.

Once done, add the billable transaction when you create the invoice. In case you haven't enabled the billable expenses feature, just follow the steps in this article: How to Enter Billable Expenses.

I'm also adding this article in case you want to learn how to personalize your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Don't hesitate to click the Reply button if you've got additional questions about converting invoice to purchase order and vice versa. I'll be here to lend some help. Have a great day!