I am Halachically considered Sephardic because my father is Sephardic (although my mother is not). I grew up in Ashkenazic schools and always davened Ashkenazic. Yet for Rosh Hashana (and sometimes Yom Kippur) my family goes away to a Sephardic shul and I daven in the Sephardic nussach. Is this proper to switch back and forth?

It is very important that one should say prayers one is used to. Therefore, switching Nusach is not advisable. You should try to daven regularly with your Nusach, unless the davening is completely different in which case you can switch. Rabbi Yitzchak Grinblat