Officially made January 31 the end date for dropped MM companions.
"Your old companion will continue to receive your eligible Premier status through January 31 of the next year. Starting February 1 their status will be based on their individual account activity."
New companion starts in early January.
"Yes, any change to your companion selection must be completed between September 1 and November 30 of any year, and such change will take effect the following calendar year (during early January)."