From the Edit Order FAQ,
You might not be able to edit an order for the following reasons:
You don't have the Edit orders permission.
The order was placed in a different currency to your store's currency. If your store sells in multiple currencies, then you can edit only orders that are in your store's currency.
The order is more than 60 days old.
I am now using the Edit Order API but I'm having the same caveat in my app. I can't edit some orders in another currency. Is there any way to work around this limit?