By default, Salesforce has only one currency set when the Salesforce contract is signed. If you need to use multiple currencies, you will have to activate them and set the conversion rate for each relative to the corporate’s default currency.
If multiple currencies are used in the CRM, it is strongly advised to enable advanced currency management in order to add dated exchange rates and thus define the exchange rate date ranges. Exchange rate management is manual.
When you change the conversion rates on a simple currency management, all currency amounts are updated using the new rates (closed opportunities, reports, history …)