the transition from class to class does not lose you the "surplus" memb miles. you start with 0 as a 'blue', if you have then acquired say 2900 miles (ie just below the next threshold) and then get 1200 memb miles on your next flight taking you over the 'blue plus' threshold you get your memb miles reset to 0 and your membership year reset to that date, and a credit to the memb miles of the upgrade surplus of 1100 miles to start your earning cycle to silver. same if you then cross the 16000 miles needed for silver, the extra surplus is carried across to start counting for gold. your membership year also gets reset. so every time you change status you start a new year of membership in that status.

if you fail to retain the points for that status in the year, you get a 'soft landing' down one grade of class in the next year from your renewal date.

i got gold for internal uk commuting, and has already been posted, you just need to work out your pattern of flying to see if it will make the class membership miles. the multiplication and division has been helpfully done for you by NickB^

hope that helps make it clearer.