Regarding the switching of a Nusach and the Hatarat Nedarim procedure. Would the same rules apply if one were to begin davening Nusach Ashkanez in order to conform to his shuls Nusach and as a result no longer daven Nusach Ari? Also should one tell his Rabbi that he has switched Nusachs in order to undergo Hatarat Nedarim?
Switching one's Nusach in conformity with one's Shul Nusach would not requrie Hatarat Nedarim at all, for this comes under the category of "Makom Shenahagu" ( Mishnah,Peschaim 4:1). The gist of this Halacha is that anyone relocating from an area observing on minhag to a locale with a different minhag, one automatically assumes the new Minhag - if indeed there is a sense of "Kehillah" and all Minhagim are binding on all the member families. Membership in an Ashkenazi Shul, at least for the purposes of Nusach HaTefilla, would constitute such a situation making relevant the Mishnah in Pesachim 4:1. Therefore, nothing special has to be told to your Rabbi.