Lets say you have grunt unit. And you attached sword on it
( to make an attachment you need to create new custom ability, like "armor bonus +1", remove all stats and add attachment points and model to that ability, then you just give ability to unit, search for tutorials if you dont know how to do that).
Now you want to morph sword to like, flaming sword.
Now whatever triggers your morphing. Lets say when you pick up some kind of fire orb(item). So when your grunt picks up that fire orb, you will remove sword that is attached to him by triggers (GUI=
EVENT-units aquired item
CONDITION-what item(fire orb)
ACTION-remove ability(sword attachment from unit(grunt),
give ability(special effect attachment) to unit(grunt) [this will give you special effect, and you have to remove it then]
give ability(flaming sword)
I hope you understood me, if you didnt I will make you an example map...
And I think that this is what you were looking for ?