OK, so the poem is called The Metamorphoses; it doesn't take a genius to figure out that "Transformation" is going to be the most important theme. That said, you might be surprised by the wide range of transformations that happen in Ovid's book. The most obvious, of course, are the physical transformations, in which a living being or material object acquires a new form. This often happens through supernatural means, such as – to take one example out of very, very many – when the nymph Daphne is transformed into a laurel tree. That said, Ovid also includes a range of other types of transformations, including changes in human culture (such as in the changing meanings of words) or in the natural world (such as through floods, or when a creature goes through a unique life-cycle). Ovid's most basic point, which he puts in the mouth of the philosopher Pythagoras, is that the only constant in the universe is change. In light of this, it makes sense that his own definition of "transformation" keeps transforming.
Ovid portrays transformation as just as natural as it is supernatural.
Ovid uses transformation as a metaphor for poetic invention.