It is possible to change/upgrade your tickets! You can do so via your MyOrder page. You can find this option by following the next steps:

• Go to the button "manage order" in the confirmation email. 
• Click on “More Actions” and select "Change tickets". 
• Select the ticket you would like to change. The price of this ticket will be converted into credit which can be used to buy the new ticket
• Please keep in mind that if the new ticket is of a lower value than your credit, the difference in price cannot be refunded.
• Once the change is complete, you will receive a new order number with the new ticket(s)