Upgrading units (to better and different units)

[Muze]

I can't find this anywhere in the manual. What does it take to upgrade a unit (not promote) ? Seems like I am very limited in the ability to do this, and usually the option is greyed out for the unit.

 

[VimesUK]

Could it be that a barracks is needed in the city for it to work...?

 

[turdish]

AFAIK you dont need a barracks to upgrade units. Maybe you dont have the money/resource in your trade route. Try rolling your cursor over the icon, it may give you a hint (anything in red). And also units can be upgraded anywhere within your borders

 

[KristoferX]

If I'm not mistaken, if the unit appears but is greyed out then it means you have a barracks, but you don't have either enough money, or the appropriate resource to upgrade to the new unit.