In ninth and tenth grade, I was very involved in both STEM and music extracurriculars. I took AP Music Theory, regular music courses during the school year (only because they were required to participate in marching band), and every music ensemble after school (jazz band, pit orchestra, etc). Now as a sophomore going into junior year, I realized that I lost my love for music and want to focus on my STEM activities, since that is what I am passionate about. Will this look bad to college admissions officers who will see my transcript and see that I have taken music courses my first two years of high school? Will it look bad that the majority of my extracurriculars will be STEM and I will have no variety? Will it look bad that I started all of my STEM extracurriculars as a sophomore in high school, rather than as a freshman? (I don’t know if this helps, but I am a female of Pacific Islander origin)
My D dropped volleyball after sophomore year to do theater year round and add more STEM activities. I don’t think it hurt her in the admission process.
IMO, doing what you like is more important than anything else. Don’t do something just because you think it will look better to colleges.
That is fine. Part of what HS is about should be trying different things and finding your passions/interests. Do the things that excite you and work towards making a positive impact and possibly getting leadership role(s) as an upperclassman.
Just be ready if someone asks you about it, and if you have a sound reason then it’s fine. “I wanted to focus on my STEM stuff” tells a story about the evolution of what you’re interested in. (“I just wanted to go smoke pot in the wrestling room” also tells a story, but most folks might not call it a great choice.)
Also, don’t hesitate to be yourself. If you’re worried about the perfect Asian girl violin-playing scientist archetype then perhaps you might not want to focus on it so closely. Try theater or ultimate frisbee or something else that gets you up and exploring, as @happy1 suggested.
quiting ECs and replacing them with nothing looks bad. Quiting ECs to focus on the ones you are most interested in is fine. Few kids know at age 14 what their interests will be until they try.