How do I do a unit change upgrade?

I want to have Ogre Warriors upgrade to Ogre Magis like in WC2. I want the upgrade to change the icon in the Ogre Mound and change existing units in the game like it does when upgrade the Siege Engine. I tried to copy the upgrade for the Siege Engine and replace the uint dependencies but the game shut down when the upgrade finishes in-game. Anyone know how this is done? Also, any things you would like to see from WC2 brought back besides Ogres, Trolls and Dragons becasue I already redid those. I also brought bacl elven archers, but I need human archers so I can have Elven archers upgrade to them. Input is appreciated.

Use the chaos ability, and have the requirements be your upgrade. Change the new unit type to ogre-mage. That'll do it.