This is not possible. If you reached a rank before merit system, you cannot loose that rank.
That is not correct.
If you are rank which is
under Hero member and you delete your posts, your activity will drop and if it drops to previous rank level your rank will drop too.
OP was Full member as you can see from his avatar, either he or mod deleted his old posts and his rank dropped to member, unfortunately for OP and judging his activity that happened maybe a week before merit was introduced.
I know that because the same thing happened to me, I was Senior member only 3 days before merit was introduced
OP there is nothing you can to, be constructive and eventually you will get enough merits to rank up.