I think that is the problem, figuring out how to strike the balance. Thaumatergic shapeshifting feels like it should be one of the cool but hard things to do. I know I am trying to figure out how to make it work and keep it hard enough to be worth it.
It seems like it should be harder then I take you out without your resisting, and easier then a death spell/tranform fully resisiting other person, and from a game persepctice, a more impresssive/powerful from should be a bit harder (though some of that can be handled by making the rolls and tasks to get the Lore Bonuses to be harder. Dog stuff is not as hard to find as wolf for example, pigeon is easy, bald eagle is less so).