If you go to the following page, http://www.chess.com/membership?c=account, it should show a credit amount (I think that is the page). If you upgrade you will be credited that amount towards your new membership level/duration, or at least, that is how I understand that it should work.
Upgrading chess.com membership
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This app is not free.
So on september 17, my chess.com membership renewed itself. Now I have a monthly membership, but I would like to upgrade it to a yearly membership. How will the money work? Would it be like I paid for the monthly and yearly? Or will it just add the diffrence of the yearly and monthly to my credit card as if I never had a monthly membership and only a yearly membership?