I am separated from my family of origin, and the only practicing Jew in my entire family; everyone else converted out. Given my family is Bohemian, I assume they davened nusach Ashkenaz, but I do not know for sure. In my youth, I taught myself to daven using a Chabad Nusach Ari siddur. I chose that siddur because it was the only Orthodox siddur I could locate. Later, I was blessed to attend yeshiva for many years; they davened Ashkenaz, and I was instructed to follow that nusach. I did as instructed, but continued to feel drawn to the Nusach Ari. Now, many decades later, I am still drawn to the Nusach Ari, and so am considering a return. The nusach seems "just right" (except for the Bedtime Krias Shema). Am I permitted to change back to the Nusach Ari? If I am and do, would I be required to change my overall minhag as well? Although mystically inclined, I am quite solidly yeshivish, definitely not a chassid. Given I am without family, I do worry that, if I daven Nusach Ari while still clearly yeshivish in orientation, I will become even more isolated within Yiddishkeit, fitting in neither here nor there. Any guidance you might offer would be very helpful. Thank you, and kol tuv.

ב"ה Shalom, As I have written previously to other similar questions, a person should not feel his Tefilla is incomplete when davening in one Nusach and not another. The Ari z"l said that in the heaven there are 12 windows (or gates) corresponding to each tribe and the Tefilla for each tribe is attuned to the Neshama of each tribe. (See Mishna b'rura 68:4) One should not feel that the Tefilla of one nusach outdoes another, (Shut Chatam sofer Orach Chaim 15-16) but should feel comfortable with what he is davening." The general rule is that a person must follow the custom of his parents. The Pasuk says "Do not abandon the 'torah' of your mother" (Mishle 1:8) which the Talmud explains that one should not change the nusach of his ancestors. (Yerushalmi Eruvin chapter 3). However, since you are not aware of the Nusach of your ancestors, your original choosing of Nusach Ari or Sefarad was fine, however I cannot intervene with the decision of the Yeshiva you were in. However, since you feel a closeness to the Nusach Ari, there are those who say that one may change to Nusach of the Ari z"l from Ashkenaz. (See Divrei Chaim 2:8 Maharam Shik, Orach Chaim 23 and Choshen mishpat 24). However, you should not go back and forth and it can also cause great confusion. Davening in Nusach Sefard does not make you a Chassid unless you choose to be. However, following minhag sefarad also has implications on the way you put on Tefillin. Do you wrap your Tefillin inward or outward? What is the knot like on Tefillin Shel rosh. These things should be consistent. Although, I have voiced some opinions on the issues of Nusach, regarding these issues it always best to go to your local Rabbi who knows you best to guide you. All the best of health and a Happy Pessach.