For example, can an Explorer Magician become a Warrior or can Aran become a Thief?
The short answer is no. Job advancements in MapleStory are class specific. A Magician can’t become a Warrior; Aran can’t become a Thief.
Basic Explorers can choose from one of five jobs at an early level (Warrior, Bowman, Thief, Magician, or Pirate), but once that base job is chosen, it can’t be changed until you complete your 4th job advancement. You’ll only be able to choose from the job advancements available to the job you’ve chosen. Job advancements don’t overlap between base classes.
When you play the other classes (Cygnus Knight, Heroes, etc.), you start with a base job already chosen. Instead of choosing from branching job advancements like Explorer classes do at level 10, you start at a higher level and move forward with your initial job class.
To learn more about classes and jobs in MapleStory, see the Class and Job Guide.