Katana updates the currency rates daily for open orders, but the rate is fixed in certain situations.
From a business perspective, the currency rate should typically be fixed at the moment an invoice is created for a customer. Katana does not have an invoicing module, but we fix the currency rates on orders in Katana at the moment when you are likely to create the invoice in your accounting system or e-commerce.
Katana will automatically fix your currency rate if:
The Sales order (SO) is set to Packed or Delivered.
Sometimes it may still be required to manually edit the conversion rate which you can learn more about here.