Would you just purchase the beast change power in addition to thaumaturgy? Can this be done with any thaumaturgical effect?
Yep, just buy the power and flavor it as very powerful spellcasting. Take Listens-to-Wind as an example, he almost certainly has True Shapeshifting. In other words, he's such a practiced shapeshifter he can change shapes to nearly anything at will and without incurring stress.