Do old orders change when the currency changes?

Answer

No. Every order stores its transaction currency at creation, and the thank-you page, tracking, and CRM render it in its original currency no matter how store settings change later.

What to do

1. Nothing: the behavior is automatic for all orders.
2. When reviewing reports after any change, trust each order's shown currency, not the store's current currency.
3. If you see a symbol mismatching the currency on an old screen, refresh; the source of truth is the currency code stored on the order.