you can always retrain, atleast i thought. but this was pre-patch thing i dont play the crusade anymore. i'm now busy testing Protectorates.

already found some bugs.
1 you cant build forts in your protectorates country
2 if you have 3 protectorates you'll get 100 million a turn
3 they won't attack countries anymore

but also some good things
1 you won't loose money anymore
2 they pay money every turn
3 they can't attack you anymore
4 thay accept almost everything you ask them