Unit 'Upgrade transformation', how?

Ok, I got the idea to make the Fel Hound for the demon race (duh really, but), and give them an upgrade that makes them Fel Stalkers, the blue version. how would i make an ugprade that does this? I checked the Berserker Upgrade, but it doesnt seem modifiable.

*kicks topic*

yo i used the desroyer form herers the map i use it for testing spells

you see, I don't want a 'pay to convert', I want all units to be retroactively swapped for the new one, like when you research Berserker Upgrade for Troll Headhunters.

Base it off of Berserk, and modify the Berserker upgrade in the 'Abilities' tab, and there is a column that says new unit type.

aha! thanks much.