I didn;t fill out any credit card details, by the way
Membership out of nowhere

Members can also gift memberships to other members. You might be able to see what happened by visiting https://www.chess.com/settings/account-history.

Members can also gift memberships to other members. You might be able to see what happened by visiting https://www.chess.com/settings/account-history.
I have a gold membership and I want to change for a day to platinum. Where is the option? Because they said you can do that. In membership, it only gives me the option to pay, not to upgrade/downgrade, and in your link, I can see the history but when I go to membership plans, is the same page that before. Can you please help me?

I don't know if the new site automatically will apply a balance or not. You might be best off opening a ticket to ask.
https://support.chess.com/customer/portal/emails/new

On the membership page, https://www.chess.com/membership, you should select the Platinum membership plan. On the next page, when you see payment options, you should be able to select to use your "membership credit," which is based on your existing membership that you have already paid for, instead of paying anything more. However, I have never done this myself, so I'm not 100% sure how it works. As Martin said, I would suggest that if you don't want to take any risk, you can contact support and they should be able to help you very easily.
Regards!

The thing is that the option to use my credit for upgrades/downgrades is not there anymore.
Last time I asked how to do it, instead of telling me, they just did it for me (is what I wanted, but I didn't ask them to do it). I want to know how to do it myself so I am not depending on anyone (and I do it when I want to do it) and that's why I asked here first. Hopefully a staff member will read it and answer me (or fix the membership page).

post 7 IS a staff member. Give him a minute, he might be back. See the little pawn next to his name?
https://www.chess.com/forum/view/general/how-am-i-a-diamond-member?page=1#comment