Hmm. I'll just go through the entire screen:



Character - Obvious, this is the character who's class you want to change.
Class - Again, obvious. The class you want the Character to change into.

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Level:

( ) Retain current level : You'll stay the same level - but EXP will be reset, as though you just reached that level.
( ) Character Becomes Level 1: No explanation needed.

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Learned Skills:

( ) No change in skills: You'll have the same skills as you did before. Any skills that you want to learn, you'll have to reach the level that you specified.
( ) Add Skills up to current level in New Class and Delete Old Skills: You'll lose all the skills you had, but you'll gain the abilities of the new class instantly.
( ) Add Skills up to current level in New Class to Old Skills: You'll keep your old skills, and you'll gain the skills of the new class instantly.

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Base Statistics:

( ) No Change in Statistics - You'll retain the same statistics. This is like the Final Fantasy Tactics Advance system I mentioned, where any changes to your class are permanent.
( ) Halve Statistics - You probably won't use this one often.
( ) Change Statistics to Level 1 in New Class - Useful only if you're changing the level to 1 at the same time.. otherwise, you'll be level 49 with level 1 stats..
( ) Change Statistics to Current Level in New Class - The one you'll use most often. If you switch from Fighter to Black Mage, you might have your strength decrease and Magic increased.

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( ) Show level-up message if Level Differs after class change - This one is useless. You'll never level up - you'll either retain your level or set it to 1. I have no idea why this is there.

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As KagomJack mentioned, I built him a class changing thing - I'll have to upload it somewhere or catch you on AIM for you to mess around with it.