I take a different viewpoint, @eko. While you are right in one way that exchange rates are not directly part of the financial transactions, they have a very signifiant impact. In particular, when a transaction is posted to a receivable or payable in a foreign currency from a cash or bank account in the same currency, Accounts receivable and Accounts payable balances depend on the exchange rates. If those exchange rates are later changed, all reports that depend on them change.
It does not seem like good practice to me to allow financial statements from locked periods to be altered. In fact, @lubos’ exact words on this topic were: