I'm not sure how often they change, but they can vary periodically. The rewards also vary when the entry cost changes. I believe it's to give some variety, while not splitting the player base. Typically the change is from tickets to tokens. The top prize for token tournaments is normally tickets, which then lets you enter the ticket tournaments where you can earn packs.
Many players only focus on one format, so if they ran out of tickets playing that format's tournament mode they wouldn't be able to enter tournaments ever again. (Without cash or in game ladder prizes) By switching the cost from ticket to token and the prize from pack to ticket, it ensures that players who want to play in tournaments will be able to cycle their tokens into tickets, and then their tickets into possible tournament wins, which give out a higher pack count for the cost if you win.
It's annoying when you have the tickets and want to enter pack tournaments, but as a player I can appreciate enabling token cycling into tickets periodically for those who don't stockpile tickets.