As you have maybe already noticed, the exchange desk of Tomorrowland opened a few days ago.
If you cannot or don’t want to attend the festival anymore, you can list your tickets in the Exchange desk. As soon as it is sold to someone who registerded to the waiting list, you will get a refund (after a few weeks).
This is valid for the regular tickets only, not for the Global Journey packages.
Only the main buyer is able to list the tickets on the exchange desk. And there is a 5€ refund fee per ticket.
So if you registered on the waiting list on time, soon you might receive a mail telling that some tickets are available again.