I agree with hunter56, except on for first upgrade on something, lets say a goblin, you have 2 classes that you can upgrade, you can make a goblins a soilder or a scout, scouts get faster attacks and movement with longer range but less damage. they would also get light armor and a longer weapon. When you choose to make them soilders, they get stronger, more health and shorter range in damage. they have metal armor on with a helmet or something and get a spiked club. Again, as they are upgraded they look cooler and tougher
Wow, I should add an export option for that! ;)