Upgrading units

[gesaugen]

How can I upgrade units?
I have few preachers, and in swamp at Paladin's house where I can buy zealots, if I go in info screen of my preachers I can see option for upgrade but it is grayed out...
Also, I've noticed that I have skill in "Mind section" for upgrading preachers to zealots (I play as warrior) but it doesn't have sense for me to have it if I'm trying to upgrade preachers at Paladins house...

help?