okay well....I found it a little weird that Fuuka's epiphany that transformed Lucia to Juno was...a little lackluster compared to the others.
But the more I thought about it, it fits.
Because Fuuka's need to belong with others is understandable. Japan has strict gender roles so the fact that Fuuka can't cook (at the time) and is more into technology makes her feel like she's not really the 'girly girl' (Yamato Nadeshiko) her parents wanted. In addition, she doesn't want to become a doctor when most of her family is is frustrating. Since family is everything in collectivistic cultures, she doesn't want to disappoint her parents or her family. So with these issues, she is struggling with what she is doing to contribute to society.
I'm also going to assume that she never really had any meaningful friendships as we rarely see her with other people outside of SEES and Natsuki. She doesn't even talk about other people either outside of her parents. She doesn't really have an identity with a certain group and thus labeled her as anti-social when that isn't the case. Her parents focused education on her and it is a big thing in Japan. She felt like she was alone.
So when she entered into SEES, she was willing to do something to help other people and get away from her family too. So when she knew the members of SEES even more she realized "Oh hey, I'm not the only one with family problems! Some of them are even worse!" And thus, making her have a sense of belonging to others.
However, while she's best friends with Natsuki, she never disclosed to Fuuka about her family situation. So hearing your best friend telling you that "hey, you have it good. Make use of it" is enough for the girl. Plus, when Natsuki tells Fuuka to screw what other people think is a major change as she is traditional and in Japan, having what others think about you is something major. So with realizing that she isn't alone anymore and wanting to stay with her friends is what caused the change.
and that I realized why I struggled with Fuuka's passiveness. while I play her as someone who is taking her time to go talk to people...it's just that she lacks assertiveness. and confrontation in Japan? A big giant no! Which goes with why Fuuka is rarely angry compared to the others In Japan? If you want to confront someone? You need to talk to someone else as a mediator. So while Fuuka realized that she needs to be more assertive? Willing to be assertive and quickly changing your passiveness is another. With the events of FES, it shows that she is slowly getting more comfortable with speaking out though.