OK, I've edited everyone's character sheets so they have all the important information, and are in the same general format. That should make things clearer!
Everyone has to complete their basic career (that is, buy all the characteristic advances available to them, at 100 exp for each) before they can change career. This means Mortus will be able to change before the others, because wizard's apprentice has a very small advance scheme.
It costs 100 exp to change to a career listed in your career exits (may be a basic or an advanced career). Or you can choose another basic career from the same class (so another academic career if an academic) for 100 exp. Or a basic career from a different class for 200 exp.
The new class will have a new advance scheme, new skills, and new equipment you will need. You don't start with any of those!