Some clarifications and recommendations:
Unless you have pretty good gears it is recommended you do not change your novice as soon as you hit job level 10, as the novice gears will make you stronger than a first job character is at first and will therefore allow you to level up more quickly than if you change right away.
Generally you can change job for any class at job level 40 (some of the extended classes have different requirements) but some classes are better off going to job level 50, either because they get certain bonus stats at job level 50 or because the extra skill points are necessary (priest/monk class is one example where this is recommended) - and I recommend always going to job level 50 on first class trans chars. Getting to job level 50 takes more time and work though, and for many classes it is not strictly necessary.