In a story I'm about to start, magic is an inherited ability to manipulate energy and matter. Mages use mage sight to see elements, compounds etc. as different colours and energy as distortions, then manipulate them to 'cast spells'- but they don't have to understand the physics, chemistry or biology to do so, they just know how to do it(they know how to use oxygen and energy to create fire, how to use water in the air as ice-blades by turning heat energy to light+kinetic, that kind of stuff).
In this setting-with all laws of physics- how would shapeshifting work? In my head, all mages need a dna 'scan' from several species and have three distinct alternate forms-aquatic,aerial and terrestrial- while Druids(one of many variants, like mindreader or illusionist) can add more, with greater variance(amphibious, half human, that kinda stuff)