[General] Trigger which replaces ability with another through holding an item

[FaceJaraxxus]

Hi! I'm trying to make so called "Heirloom" items for specific heroes. So when the hero picks up the item, and conditions are true, one of its hero abilities will be replaced with the heirloom ability of the same level. Of course, the hero can level it, so when the item is dropped, and conditions are true, the original ability is returned with the same level as the heirloom ability. The Heirloom ability is of course removed (not the hard part).

For starters, I've tried making a SetVariable action to identify the manipulating hero's level of the specific ability. This is also where it stops. Despite going through a Real Level Field guide I'm unable to move forward. This is foreign territory to me so please help me!