I think you need to get to the reason that she switched you in the first place. I don't know anything about your singing, or the choir you're singing in, so I can't say for sure. My wife's a choir director, though, and here are some reasons why she might move someone from one section to another
1) To improve that person's musicianship (as you're aware, it's a different world to sing a soprano part than an alto part, or a tenor part than a bass part)
2) To improve the balance between the sections. Maybe the sopranos are being overpowered, so the director puts a good musician on the soprano part to help balance things out.
3) To give the section a leader. If the soprano section doesn't read well, then maybe the director needs you in there to help pull them along.
4) To improve the blend of the sections. Maybe in a C3 (wow!) to C5 range, your voice has a timbre that doesn't blend as well with the other alto singers, as it does in the C4-A5 range with the sopranos.
5) To improve non-musical issues in the choir. If it's not a professional choir, then the director has to deal with behavior issues, personality clashes within sections, and all those things that you'd HOPE were left behind when you graduated from middle school, but will still be present all the way through your college ensembles... :-(
It could be any number of things. Choir directors change voice parts for lots of reasons, especially when you've got a singer with such a wide range. Now, what's weird is that you're being switched FROM alto TO 1st soprano. Seems like the switch would normally happen the other way...where your choir has 1000 girls/women who can sing the 1st soprano part, and 5 who can sing the alto part.
There is another possibility, of course. I've been giving your choir director the benefit of the doubt, which, not knowing her as a musician, I think she deserves from me. But there is the possibility that she doesn't know what she's doing. Maybe her logic is "boy, I've really gotta get more people on the 1st soprano part because that's the melody, and that should be the loudest part." (I totally disagree with that philosophy, but there are some who believe it). In which case, she'll think "I've got this alto who can sing at the top of the staff...she should be a first soprano, too!"
My advice would be to sit down and talk to the director in a very non-confrontational manner. Tell her that you're feeling some vocal discomfort by constantly being asked to sing at the top of your (non-whistle) range. Obviously, there's a difference between "I can sing up to a B5," and "I can comfortably sing music that sits between F5-B5." Your director should understand that. Tell her you're feeling discomfort, and ask if she could help you understand the reason why she moved you.