Still musing here....Don't try this at home folks, I'm an semi-trained writer with no insurance! This is another stream of thought, done on the fly, and will probably embarrass me sometime down the road.....
There are two forms of energy Order Magic users can call on, for different purposes. Manna and Chi. Both these energy forms are in the environment around the magic user, and they dictate what spells can be used.
Manna is energy given off by the classic basic elements --- Fire, Water, Earth, and Air. There are millions of smaller sources of manna, as every non-living thing on the planet has some manna in it, but everything falls under one of the four elements. Under most circumstances, a wizard or witch will use manna from the basic elements for spells.
Earth and air manna are the most common, and are readily available for a human magical user's use. Fire and water need a source to be useful, but the more powerful and experienced a wizard is, the less manna they need to work their spells with. So, while a novice or weak magic user might need a bonfire or a lake to create the spell, a master wizard could get by with a recently put out campfire or a puddle to create the same spell. Most attack and defense spells are manna-based.
As I mentioned before, all non-living material gives off manna, but in small quantities compared to the major elements. Items like rocks, metal, plastic, sand, glass, all give off manna. However, raw material gives off more manna than refined material, so an untouched granite bolder gives off more manna than a granite statue of the same weight. A wizard will use these other items for various reasons, including wards and to add extra energy to a spell. Gemstones are prized for their manna, and most wizards will carry a few for their use in spells.
Chi is the energy given off by all living things. Like with Manna, living (raw) life has more energy than dead (refined) life. The amount of energy given off is based on size, length of life, and intelligence of the lifeform. Humans give off more chi than most animals.
Chi is trickier to use than manna. For one, most magic user use only their own chi. Using someone else chi without their permission is considered a series breach of trust. Necromancy is using spirit chi to control spirits of the dead, and is one of the few things that will cause the magic users community to band together and hunt the Necromancer down.
Second, the spells that need chi are limited. Most Chi-based spells are for things like strengthening the human body, healing wounds, or pushing the human body to over its limits. A magic user doesn't have enough chi to use it like a fireball or force bolt spell. Any chi spells are either internal to the wizard/witch or delivered by touch.
While it would be theoretically possible to change a human into an animal, the amount of time and animal chi needed would be too high to sustain the change. The more extreme a change is, the more chi it takes, as the chi of the target needs to be overwhelmed to force the change.
That's enough for now -- now I have the weekend to do some more thinking....