Just like guilds, players can leave a union and join another union anytime. However, there is a 48-hour cooldown between leaving and joining another union. In addition, players must be at least level 50 in order to join a union.
Joining a Union
There are currently 6 unions available for adventurers to join. You do not have to be a job class of that union in order to join (i.e. a Warrior can join the Hunter Union).
Once you join a union, you can earn Union Points, which can be spent on exclusive permanent costumes among other useful items.