If you want to give a unit a base model without editting it in the object editor, you can import a model and set its custom path to exactly the same as the normal model path.
That way, it overrides the old model.
But you can simply go to the object editor and give it the right model if you want.
However for temporary models (like avatar, metamorphosis, etc), the easiest thing to do is make an ability based of either of those abilities and change the normal and alternative unit.