I acknowledge that your client would like to convert invoices into a quote, @KerynC.

Allow me to chime in and share some about this in QuickBooks Online (QBO).

In QBO, converting invoices into a quote is unavailable. The standard process is to create a quote first and then convert it into an invoice when the customer accepts the quote.

With this, You'll need to delete the invoice and create a quote.

To delete an invoice:

1. Find the second invoice and open the transaction.
2. Click More at the bottom and select Delete.
3. Hit Yes to confirm.

Once deleted, you can follow the steps below to re-create the invoice from the quote.

1. Hit + New, then click Quote.
2. Select Add Customer. Then choose a customer from the ▼ dropdown. Or, to add a new customer, hit + Add new, enter the customer info, and select Save.
3. Tap the ▼ dropdown in the Product or service field and add the products or services to the quote. Or, to add a new product or service, select + Add new, enter the info for your product or service, and click Save and close.
   
    

For your reference, you can check out this article on how to convert a quote into an invoice in QuickBooks Online using web browser. 

Additionally, here's an article to help you manage invoice payments: Record invoice payments in QuickBooks Online.

Please don't hesitate to get back to me if you need further assistance with this. I'll be here, ready to help you, @KerynC. Take care, and have a good one!