Igros Moshe Orach Chaim 4:24
"Behold it is known that all the people of Poland, Hungary, and Russia...are children of Ashkenaz even Chasidim. And until Chasidus spread they all prayed with nusach Ashkenaz. However, the Chasidic leaders lead them to pray in a different nusach with various changes...They changed the customs of their ancestors and our great rabbis of Germany and France. The reason for the change is not clear nor how they permitted a change from the established nusach....If one desires to change back and pray in nusach Ashkenaz, since it is the nusach of our ancestors and rabbis, he is permitted as he is returning to what once was.