Roll your cursor up to Home in the upper right and click on membership. You should see something like the following which will tell you how much membership credit you have. It won't be quite a full month's worth because they'll deduct a day or two, but whatever that amount is will be deducted from your yearly charge on your credit card. Hope that helps.
Platinum
GoldYou have $27.67 of membership credit available for upgrade
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?