Listen, I'll tell you right away that it's not complicated. I'm just afraid that I haven't explained anything to anyone in a long time, I've forgotten everything, and I've lost my touch, so I'll keep it very short and not very detailed. And yes, there are at least two options, and I'll use the map editor.
To replace the initial abilities, it's easier to do it this way.
Create a new map and place the desired hero there. Left-click on him and go to the left navigation panel with the object properties tree (ObjectPropretiesTree). There, you need to find "Shared" and click on it. Then, a button "..." will appear right next to the label; click on it.
And there, a dialog menu will open. You need to remember a little English and find the hero's skills in this list and simply change them.
After all these steps, save the map, and then extract the map from it (open it with WinRAR, delete the "Maps" folder inside the archive). Rename the archive from *.h5m to *.h5u, and you're done.

And the second option is to create a copy of this hero with a model and assign it to the Dark faction with all the bonuses, such as enhanced spells (redesign the talents for the Dark faction, but leave the model).

There, select the class and the desired abilities.