According to another post from someone on staff, you don't need any help to downgrade. Just click on the plan you want, confirm it, and it's automatically taken care of. Haven't tried it my self, but sounds logical. Here's the link:
http://www.chess.com/membership?c=navbar
Now as to deleting that second account just go here and right under the blue Facebook thingy is a very small print sentence you click on.
http://www.chess.com/home/account
I have two accounts and I want to delete one. I have a diamond membership and would like to downgrade to platinum