In Medieval II: Total War your units change as you upgrade them.
The basic spear is just a stick with a pointed metal tip, the upgrade gives it barbs. Basic armor is just padded jackets, but upgrading them to chain mail gives them... chain mail! Two-handed swords first look dull and heavy, at tier 2 they look shiny and well-crafted, then at tier 3 they get wavy blades making them Flamberges.
It really is a great game.