This question is actually a broader one, and is addressed in many of the tshuvos sforim of yesteryear. I cannot cite the exact reference, but the Divrei Chaim of Tzanz expounds on this issue. The Ashkenazic Jews had been davening what is known as Nusach Ashkenaz (presumably that which is labeled as such today). The adoption of Nusach Sfard (definitely not the nuschaos used by various kehillos of Sephardic Jews) is questioned as a departure. According to the Divrei Chaim, Nusach Ha’Ari is an inclusive nusach, and is avaiable to anyone. There are several different kabalos on what the exact Nusach Ha’Ari was. The siddur used by Chabad is labeled Nusach Ha’Ari, but is definitely not. It is the nusach compiled by the Baal Hatanya, based on Nusach Ha’Ari. There are glaring differences. In any case, it is not considered a departure to daven a general nusach, especially one based on the kabala of the Ari.
What remains a serious question is the marketed Nusach Sfard. It seems clear that the contemporary Sfard is a conglomeration of whatever the publisher decided to include. There are many proofs to this (references available). The common Nusach Ashkenaz is probably more accurate, but also not precise.