Chapter 160- Moving Floors (Part 1)
What does Shapeshifting mean in general? Well, Shapeshifting is the ability to change the state, the form, or the appearance of something from one thing to another. In a broader sense, that means that this ability didn't care about the material this thing was made of or what they were intending to turn it to.
However, Rin knew more than anyone else that this was nothing short of absurd. The ability, as strong as it was, most likely had restraints that makes it stay within the worldly laws of Divine Sapphire. One of the restraints Rin could think of is that Sorophi could only change one thing from one state to another only if the two objects were at least close when it comes to appearance.
For example, when she turned that plushy into a huge bear, the two things were quite close when it comes to appearance. The same thing applies to her child form and adult form. Rin was certain that this was the case even without asking Sorophi about it.