Actually the job system doesnt work quite that way.
Once you get to a novice with job level 10 you can become either an extended class or one of the 1st classes. If you decide to become an extended class (like a ninja for example) there is no more job changing.
If you become one of the first classes (a mage for example) once you get to level 40-50 (You can change from anywhere between 40 and 50) you can become either one of the 2-1 classes or the 2-2 jobs
Whichever 2-1 or 2-2 job you become once you get to base level 99 you can "transcend" which means you basically start again but instead of becoming a 2-2 class like a crusader you'd be a paladin instead.
Example:
Novice job lvl 10 > mage job lvl 45 > wizard base level 99 > trans >
High novice job lvl 10 > high mage job lvl 50 > High wizard
Although for example say you were a wizard before you transed, you dont have to pick high wizard when you trans. I hope this helps somewhat with how the job changing system works.