Might I suggest that instead of increasing everything when you upgrade a class you choose to specialize in one of the areas. A new picture could be made for the more general class (Worker, Dueler, Soldier, Adventurer) and when you get to level 30 or so, you choose to specialize as a specific type of of that class (for the dueler Bounty Hunter, Gunslinger, Bandit Hangdog). Once the specialization is chosen, you would get the upgrade corresponding to that class.
For example at level ten a dueler gets:
Dueler:
+ 10% speed
+ 20% motivation
+ 10% more money
At Level 30 the dueler choses:
Bounty Hunter:
+20% speed (10% increase)
+ 20% motivation
+ 10% more money
Gunsliger:
+ 10% speed
+40% motivation (20% increase)
+ 10% more money
Bandit:
+ 10% speed
+ 20% motivation
+ 20% money (10% increase)
Hangdog:
+ 13% speed (3% increase)
+ 26% motivation (6% increase)
+ 13% more money (3% increase)
Also note that this would be a great way to implement special abilities in duels