How to connect an existing invoice to a PO?

I'd like to officially welcome you here in the QuickBooks Community, jsfitch! I'll take this opportunity to answer your inquiry.

You can always copy or convert a PO into a bill. However, there's no option to do it the other way around. 

You'll want to delete the bill first. Then, follow these steps on how to create a new bill from the PO:

1. Open the Purchase Order.
2. Click the Copy to Bill button beside the amount.
3. Enter all other details of the bill.
4. Choose Save and close. 

Additional details about this process are discussed here: Add Purchase Orders To Expenses, Bills, Or Checks In QuickBooks Online.

You can also check out or save these articles in case you might need them when making vendor-related transactions: 

• Learn The Difference Between Bills, Checks, And Expenses In QuickBooks Online
• Bill Created From Purchase Order Has Incorrect Or Higher Amount

If you have any further questions or need assistance when creating and paying bills, please don't hesitate to visit us again. We're here to help!