So you know how in movies whenever
mermaids turn into humans, you see that image of their tail fin splitting into
two legs, or when a person turns into a mermaid their legs fuse together to
form a tail? But like, from a evolutionary/ morphological standpoint, fishes
became tetrapods (four-legged) through development of their pelvic
fins into legs and the attachment of the pelvic girdle to the spinal column, while the fish tail developed into a tail (which is essentially an extension of the spine).
Above is a skeleton of Tiktaalik, which was the first fossil fish discovered with legs which shows the development of pelvic fins into limbs.
As we know, it takes less magic to create something out of what is already there or is closer to the end product. So really, a mermaid transforming into a person would find their vestigial pelvic fins shooting out of their sides to become giant, unwieldy limbs while their mermaid tail shrinks into oblivion.
And a human turning into a mermaid would find their legs disappearing into teeny pelvic fins while their tail bones explode out of their butt to become a giant caudal fin.
tl;dr: turning into a mermaid would not be as sexy as it seems in the movies, and should only be done in cases of extreme duress
P.S. Please excuse by crudely drawn MS Paint figures. I am grad student in Bio, not Art.