Huh? Once the Sim is a mermaid, changing them is no different than any other Sim, vampire, or alien. For basic changes (hair, clothes, makeup) you go into cas normally. For all other edits, you have to enter cheats.
You can do any genetics in cas, edits in cas, anything. But, same as a vampire, you either have to start them off as a vampire or take them in game and make them the occult (being bit to become a vamp or the kelp to become mermaid) but once they are the occult, you can do anything with them.
You just can not change occult types, Sim/human or otherwise, in cas, but you never have been able to with Sims 4.