As the OP states, even though Kohana Rin is only fifteen, she’s been working for ten years. As such, she’s by definition not your ordinary, run-of-the-mill fifteen-year-old junior high schooler. She’s a special case, which is why her counselor counsels her to seek education at a high school better suited for special cases such as hers, in which she can take acting classes and her work-related absences can be worked around.
The whole reason Rin’s been working so long (in addition to being talented) is that she used to be so painfully shy, so her parents put her in a theater trope. Since then, she’s simply gone with the flow, but it isn’t until now, when she’s now faced with going to a different school than her oldest and best friend, the heart-eyed Sayo, that she starts to doubt whether she even should be a seiyu.
The episode makes it a point to show that unlike Futaba and Ichigo, her present situation didn’t come about as a result of a choice she consciously made; her parents made it for her in hopes it would help her social skills. Futaba and Ichigo don’t lets their doubts get the best of them because they know they’re on the path they want to be on. But Rin isn’t so sure anymore.
Things become a little clearer when, suddenly and coincidentally, her manager hands her a script for an anime film where she’ll be playing the little sister of the lead, voiced by Kamiya Hiroshi (making his second appearance on SgS). The director specifically chose Rin because he wanted a real 15-year-old actor.
With few actors her age out there with as much experience, she seems the perfect choice, but Rin’s recent realization she hasn’t led a typical 15-year-old’s life makes her uncertain. The director cuts several times because she’s either sounding too responsible or too young. But that’s to be expected, considering Rin is more responsible and composed than most kids her age.
Even Kamiya tells her she impresses him; when he was fifteen, all he did was goof off, and even though he’s regarded as one of the industry’s top voices, his own opinion of himself is of someone constantly unsure if he’s even cut out to be a seiyu. He can be negative and overthink things. He never thinks he’s good enough, so he’s always polishing.
Learning this insight from such a towering voice works wonders for Rin, now that she knows she’s not the only one who feels the way she does. And however her career started, she is a seiyu, and she wants to become a great one. For that, she decides she’ll change schools for high school.
When she breaks the news to Sayo, Sayo takes it as you’d expect. She can’t hide her sadness or tears, but nor does she think it’s the end of their eternal relationship; not by a long shot. In fact, Sayo’s tears are both of sadness they won’t see as much of each other, but also joy and pride that her once-profoundly shy friend has grown so strong, and can now stand on her own two feet.
Of course, Rin still needs Sayo’s help with one thing, and will continue to year after year, no matter what: their annual end-of summer giant parfait.