DaMoshe,
“R’ Moshe zt”l wrote, there is no explanation for why the chassidim changed the text of davening, and it was a change to the text that had been used for centuries”
I’d be interested to see that teshuva inside. I thought it was well known, that the changes were to parallel the changes that the Arizal instituted in nusach sfarad.
For any CRers who may not know; The first major change by the Chassidim in nusach hatefila, was by the Alter Rebbe/Baal HaTanya. The Arizal had put in various changes to the original Sefardi nusach, and the Ba’al HaTanya copied them into nusach ashkenaz. That’s why the Lubavitcher siddur is called Nusach Ari. The later Chassidic leaders made further changes according to mystic sources, and this eventually became the Nusach sfard siddur of the chassidim today.