There are two job changes, one at lvl 15 and one at lvl 60.

Lvl 15:
Mercenary
Assist
Magician
Acrobat

Lvl 60:
Knight or Blade - Must be Mercenary
Ringmaster or Billposter - Must be Assist
Elementer or Psykeeper - Must be Magician
Jester or Ranger - Must be Acrobat