Look no further, @NewUser2021-.

Yes, you can delete and recreate the customer so you can add the correct currency. Since Multicurrency will work on future transactions after it is turned on. Existing transactions and profiles will follow the setup currency before enabling the feature. Here are the steps you can follow to fix the currency of the customer:

1. Go to the Sales menu and choose Customers.
2. Look for the customer's name you'll need to delete and check the box beside the name.
3. Hit Batch actions drop-down arrow and select Make inactive. 
4. Then, Yes. 
5. Once done, click the New customer button.
6. Enter the customer's info.
7. Go to the Payment and billing tab and pick the EUR Euro currency for the customer under the This customer pays me with the field.
8. When finished, hit Save. 

For more info about the process, please see this article: Assign a foreign currency to a customer in QuickBooks Online.

Also, to get a list of all payments and invoices for each of your customers, you may pull up the Invoices and Received Payments Report. Then, customize it so it will only show the info you need. 

Let me know how this goes and leave a reply below if you need further assistance managing currency for your customers in QBO. I'm always here to help. Have a good one.