Rider to Fel Orc Rider Upgrade

How can i do for do like the berserker upgrade (headhunter to berserker)...but with riders to fel orc riders...(the headhunter have alternate animation and the rider havn't it....) ????

I can't do that !!!:////

Ive been facing the same problem with upgrading dragon whelps to drakes and then to dragons, the icon just dosnt appear in the building menu

Just make a custom upgrade that does nothing.

Then add the chaos(raider) ability to the raider and set the techtree-requirements to the custom upgrade.