You can't turn yourself to a mermaid or change anything about the physical appearance or change genes and DNA.
~ Why isn't it possible ? ~
While casting spells the practitioner channels and manipulates energy to get his / her desire ( the practitioner may choose to do more optional things such as adding to the formula ritual supplies and tools, circles, colors, planetary hours and days, etc). But basically you can see that the practitioner manipulates energy and sends it to the universe or to another object.
It is know that simple energy manipulation and channeling will not get you any physical change. Energy is something that will not effect you genes and your DNA. That's not really how it works.
Also do not forget that the thread is talking about mermaids and not other physical changes that may be done by operations.
As I said you cannot give yourself any physical change of your body, and also you cannot change your DNA, genes and hormonal process by energy manipulation.
The thing that makes it even more impossible is that mermaids do not even exist here. They are just mythical creatures which are told in stories. But this is only fantasy stories, not reality. You need to realize what the difference is between fantasy and reality. You can join a magic site but it does not make everything to be a reality.
So If it is known that energy manipulation will not change your physical appearance, your DNA, genes or hormonal process, it is obvious that it will not turn you to anything that does not even exist but in fantasy stories.
And even if you are talking about being ' boron' one, then you probably have to read the paragraph about things that do not exist.
You were born human and will always be one.