How do Warhammer professions work? Do you just shift what skills you can learn, or is it more complex?
In 2 edition they have very strict rules - you had to add certain bonus to skills (three levels: trained, +10, +20 IIRC), you had to advance attributes to certain level compared to your starting one (so if you went through 3 professions with +10% to Melee you only advanced within first profession), and what talents. You finished it - you needed to move to next one.
4th edition changed it for better overall - because it assumes that profession is really your way of life, your trade, your social position, social duties, way you earn money.
Each profession now have 4 levels of sub-professions (in some cases it's bit unnecessary I think), and you can easily stay within your profession or advance into higher rank within the same trade like turning from Rat Catcher into professional Exterminator, or from Conscript Soldier into Sergeant of troop.
Each profession has basic skills, attributes and talents you can advance (talents are level specific, skills and attributes - you get new ones to advance on new levels while keeping all from previous levels avaliable).
There are usually no very specific things only one profession can do - aside of magic (though with some talents and other things you can do it), and you can learn skills and talents from other professions with either double XP cost - or by sacrificing one of endevours (resttime mechanics between adventures) - to get teacher and pay him - then pay normal as someone for whom it's profession skill.
And while there are certain limits in taking new rank within profession or leaving to new one - you can stay in one profession literally for ever - advancing skills, talents and attributes without upper cap. There is no more - as soldier you can uph your Melee just +10% to get +20 you need to become Veteran or something, no - these time you can just keep doing whatever job you are doing. The point is profession is not mere class it should be something your PC is really doing for life.
Zweihander sort of houseruled 2e (well very houseruled) by infamous Daniel D. Fox adds extra special talent for each profession just for them - though often it's something like free re-roll of two skills.