dtgreene: Edit: Also, from what I hear, in the 3DS version, you no longer keep all the abilities from old classes. I don't know what exactly you do and don't keep, but apparently you can keep abilities from monster classes that you can't keep from human classes.
Sarisio: Why would they change that? Worked fine in DQ VI and DW VII. Off-topic: how do vocations work in DQ IX? Can you master them and retain their abilities on switching like in DQ 6/7? How is the class (vocation?) system in comparison to DQ 6/7?
Here is how it works in Dragon Quest 9.
Levels: Each class levels up from 1 to 99. Change to a new class and you are back to level 1 with level 1 stats (but see the Skills section of this post). Change back to your old class and (unlike DQ3) you can resume from where you left off.
Spells: Each spell-casting class gets spells at certain levels. Those spells can *only* be used while you are in the class; there is no way to use a spell in a class that does not learn it naturally.
Skills: As you level up, you get skill points, like in Dragon Quest 8. Skill points are retained when you change class, as are skills you've already learned. A skill learned in one class can be learned in another, *unless* it is a weapon/shield skill for a weapon type your new class can't use, *unless* you have mastered the weapon skill, in which case you can use the weapon regardless of class. Note that there are passive skills that increase stats; get a skill of this type and it affects your stats in all classes. Note that skill points don't need to be spent in the class you learned them in.
If you level up every class to level 99, you will be 200 skill points short of mastering everything, but there is an option of "revocating", which will send you back to level 1 in a class. You can then get another 200 skill points by leveling the class to level 99 again.