I did the same thing... I called their service number 01805101111 (awfully expensive..) and explained the situation. She told me the best way is that she cancel the booking for me and I do the booking all over again. It cost nothing. She then sent me a mail about the refund, and I made a new booking for 2 persons.
You can try to call this number again, or go down to the Hbf. Apparently you won't be able to book the 35 euro ticket anymore, but you can still ask her if it is absolutely necessary that you change the ticket, and if yes how much you have to pay.