You will have to create a new bank account if there are already transactions against this account.  Once you save the account it locks in the currency.  Once you create transactions against this account it will not allow you to delete the account because there are records that require it.  

You may consider renaming the bank account to something like Do_Not_Use if it already has transactions against it.  Also, remember to update any records that use the bank account as the default so it doesn't accidentally get used in the future.