In my case, early reset is my advantage. There is no decrese of membership year. My membership years start at exactly same months past 5 or more years with early resets.

I qualified my next Diamond status before the end(e.g. 30th) of the 12th month. Then, at mid of the 12th month(e.g. 15th) of membership year, CX resets my accumulation. So I have 12 months and a few weeks for accumulating 120,000 miles to qualify another DM. So some amout of miles over 120,000 miles(flights between 15th and 30th of 12th month) can be used for the next year qualification.

If CX resets my accumulation at the end of the 12th month, those flights(e.g. 15th to 30th) can not be used for the next year qualification.