Yes! You can change your tickets directly in your order via the "manage my order" button found in your confirmation email. If you follow the link and select 'more actions', you can change your tickets. There are a number of change options and you have the freedom to use the 'credit' you get from your original tickets in any way you like.

Please keep in mind that if your new order is of a lower value than your current order, the difference in price cannot be refunded.