Tournaments - Tournament Tickets

Can I exchange my ticket for a different one with the same value?

No, it is not possible to exchange tickets at this time.

It may be possible in some cases to transfer a ticket to a friend.

It seems a tournament ticket that I had possessed is no longer in my account. What happened to my ticket?

Some tournament tickets have expiration dates such as those used to enter Freeroll tournaments. 

It is the player's discretion to check the expiration dates of tickets.

Another reason may be that the player had already used the ticket to register for a tournament that starts later. In this case, the player must check the tournament list to check whether their Nickname appears in the Player list of the pertaining tournament.