To answer your questions in order:
1- Why would you want to? Ashkenaz is an ancient mesorah approximately 3 times as old as Nusach Sfard. More, when you take into account the fact that our contemporary “nussach sfard” is a composite and not what any of the earlier rebbes davened. If you have a kesher to some particular chasidus and follow their customs it’s another story.
2- There are Mizrachi, Aram Soba, Shami, Baladi, Sfardi, S&P (aka real sefardi), various girsaos in Nusach Ha’ari l’minehem, to give a partial list.
3- You can switch havaras. Whether or not you should is another story. Your havara is (or should be) determined by the tradition in the rules of dikduk you received from your parents and/or rebbeim.