If you have one of the Treasure Hunter (Adventurer), Plainswalker (Wind Rider) and Abyss Walker (Ghost Hunter) as the sub class, you won't be able to add the other ones as your new sub class. (The ones in the brackets are for the third occupation change.)
If you have one of the Hawkeye (Sagittarius), Silver Ranger (Moonlight Sentinel) and Phantom Ranger (Ghost Sentinel) as the sub class, you won't be able to add the other ones as your new sub class. (The ones in the brackets are for the third occupation change.)