lesschumras:
I’m curious, what was the mesorah for Chassidus and their adoption of a whole new nussach and minhaggim?
The mesorah begins by our gedolei hador. They are the ones to institute or remove elements of our hanhagah according to their da’as Torah. The Besh’t is obviously included in this elite group.
(You may argue that the opposition would disagree and I would turn your attention to the quote of the Chofetz Chaim by R’ Elchonon in koyveitz ma’amarim.)