Why is the store currency locked after the first order?

Answer

After the first real order the base currency locks to protect pricing data, because changing it re-labels an entire financial history and mixes figures across currencies. You will see a notice that the store currency is locked after the first order.

What to do

1. Do not attempt to change the currency in the UI after the lock.
2. Contact support if you need a currency migration.
3. Confirm that past orders keep their original currency in reports and invoices.