My most drastic change of heart regarding a H!P member was the recent Yaguchi scandal. I know she hasn't been H!P in years, but she was one of the first people in MM I discovered back in the early 00's, so I was a fan for a
long time. I even took up translating her blog off and on for a couple years when the first person stopped. I really looked up to her because of how genuine she came across. But I lost absolutely all respect I held for her. When she left MM, I was able to see it as following her heart. But cheating on her husband, who seems like a genuinely great dude... nah, no way to justify that. Even with all the scandal and fighting Aibon has gone through, I just feel pity. I can't feel anything for Yaguchi anymore.
My next biggest was probably Sayu. I thought she was a cute character for a long time after she joined and was neither here nor there about her, but then she started with the narcissist character and I sort of drifted away (not understanding at the time that an Idol persona was different than the actual person's). But when she finally got her blog and it became easier to understand the real Sayu, I started paying more attention again. She became my Oshi when she began opening up her heart about her hardships and you could see just how fiercely she loves MM and worked to promote the group. And when she struggled with really bad acne, I was just completely endeared at how she handled it. Not to mention, her fanbase at the time was really tight-knit, so her threads were cozy places to be (but hey, I'm totally welcoming all the new people who have grown to love her too lol).
I never paid much attention to Chisato, but when she started posting those dance covers to youtube, I started paying more attention to her and just absolutely love her now
Likewise, I liked Kumai alright, but she became my Berryz fav after the Guardians 4 thing and then Ousama Game.