Kaorin was replaced by Kagura and it was for the better
Basically Kaorin didnt have enough going for her. Personality-wise, she was too much of a straight(lol) man for everyone else, which Yomi already filled, and her only real personality was her Sakakilust (understandable). Beyond that, she didn't look visually distinct enough from the others. She looked too much like osaka, too much like tomo, and especially too much like other side characters. Azuma could have corrected the personality by fleshing her out, but then he would would still have been limited by her design, which didn't stand out enough from the cast. Basically she was doomed to be a gimmick character from the start. When he decided to pivot into Kagura joining the crew, she immediately had much more going for her. Her design as a browntomboy, of course, was distinct from the rest, and she meshed well with every other character in different ways because she had more than one facet to her personality. She was apart of the knucklehead trio, she had this pseudo-rivalry with Sakaki and Tomo, she fit as this sort of protégé type girl of Nyamo. She was a support character but was supporting the whole cast instead of her personality being tied strictly to one character, like Kaorin was. This also made it really easy to expand her character into having a heart of pure gold. Kagura is by far the most emotional of the cast outside of chyio- but chyio is justified as her being younger and it's mostly played for cute moments or laughs, meanwhile Kagura's emotional vulnerability is played straight because she is just that heavy-hearted. All around it was a good decision to swap them out. Both the manga and the anime both really came into their own when Kagura was brought into the fold. I don't want to directly say that she's the glue that holds the group together, because they were together just fine without her, but she definitely helped them all grow closer just by her presence alone. Kagura is great.Kaoringay