Can I credit and rebill an invoice that has already been paid? I originally billed it as non-inventory (COGS) & it is an inventory item that needs to be reflected as sold

Yes, you can, @info1044.

You'll want to unlink the payment made to the invoice then change it to an inventory item. Let me show you how to do it in your QuickBooks Online (QBO).

Here's how:

1. Open the paid Invoice
2. Click the 1 payment made link and the date link. 
3. Uncheck the invoice to unlink it to the payment and click Clear Payment to delete the credit. 
4. Click Save and close.

Then, edit the invoice and receive the payment as you normally do.

Additionally, here's an article you read for future reference. This link can guide you through the detailed steps in personalizing your invoice: Customize invoices, estimates, and sales receipts in QuickBooks Online.

You can always get back to me whenever you have other concerns. I'll be around to help. Take care!