Are there any apps available for amending "existing" customer records using the API or is it definitely not possible?

Hello there, dwhite2.

As far as what we're doing in QuickBooks Online, we'll need to enter the transactions (such as credit memo, refunds, invoices, and etc.) manually if we want the customer balances to change. You might find these articles helpful:

• Enter customer opening balances in QuickBooks Online
• How to enter outstanding balances for customers and vendors

Meanwhile, you can click the Apps menu and search for possible applications that can help you achieve your goal. Aside from that, we also have the Intuit Developer page. There are QuickBooks users, engineers, and developers that can also share their ideas about the third-party application you want to use.

1. Go to this link and log in to your Intuit account: https://help.developer.intuit.com/s/.
2. Select QuickBooks Online under What do you need help with?.
3. Click Ask a Question and enter the details of your concern.
4. Select Ask.

You can also get back to us with more details so we can assist you the situation. We'll be around. 

@dwhite2 

You may export the data out to XLS, modify it, and import them back in bulk.

https://transactionpro.grsm.io/qbo