See here.
"...you can go to the membership page at http://www.chess.com/membership.html where you can change from one premium membership plan to another whenever you like. And when you change from one premium membership to another any remaining membership credit you currently have will be put towards the new membership plan that you are changing to."
Hi I am a chess.com basic member, and am thinking of upgrading my account soon. I can't choose whether I want platinum or diamond, so, I have a question. If for example I buy a year of platinum membership, then two months later I decide that I want diamond, do I pay the difference for the next eight months, or do I have to pay the full cost a second time?