all spells and skills from the previous class will carry over. Remember though, that Spell and HIt Points are hard coded and can't be modded: 1. If you change a Spell caster to a non-spell caster, the character will lose all spell points (but, ironically retain. the learned spells. He/she just can no longer cast them).
2. If you change a non-spell caster to a spell caster, the character will gain spell points and will be able to learn skills/spells applicable to the new class.
3. Your new professions will assume the spell and hitpoint values for the class TO WHICH YOU HAVE CHANGED. For example if you change a level 50 Champion to a Master Archer, he/she will gain the spell points of a level 50 Master Archer, but lose the Hitpoints of a level 50 Champion and the character's HP will be adjusted to those of a level 50 Master Archer. This is true for ANY cvhange you make, so plan wisely!!
Other than that everything will carr over. If again, for example, you change a Spy to a Champion, you new champion will (if you fully trained your Spy), retain GM Disarm Trap, GM Leather, Master Perception, and GM Dagger and can learn GM Plate, Sword, Spear, Armsmater, Body Building and Shield.