Basically put the base stats of both the base and "upgraded" classes are essentially the same. What differs is their equipment/magic choices. Nearly all classes gain the use of extra, more powerful equipment, as well as new spells. The only class that does not benefit at all from the class change in any way whatsoever is the Black Belt. A Level 50 Fighter, on average, will have the same base stats as a Level 50 Knight. The only thing that is dependant upon when you change classes is the MP of the Knight and Ninja. Basically they only start gaining MP from the level after the one which they upgrade on. In other words, a Knight who class-changed at Level 10 will have more MP at Level 20 than a Knight who class-changed at Level 15.