This question is an exact duplicate of:
If I am not mistaken initially there were Ashkenazic Jews which later on branched off into Chassidim via the leadership of the Baal Shem Tov.
Does this mean that prior to the Baal Shem Tov all ashkenezim had the same (or almost) the same pronunciation and then it was changed drastically by the chassidic movement?
What was the reason behind the change? Why were the rabbis of the time allowed to issue the change?