Hey janicecdn,
It's great that you've already identified this issue. QuickBooks Online's multi-currency features are a powerful and convenient way to manage your international business. This comes with advanced features built-in, providing precise tracking for your exchange gains and losses. In order to maintain consistency in reporting, an account's currency is set upon being created and each transaction is considered individually to ensure accurate foreign balances. Because of this, the best way to change an account's currency is to recreate it with the correct settings. You'll then be able to safely edit each transaction to change the account, while properly recording exchange rates.
Hope this helps!