Yes, you can go to the membership page at http://www.chess.com/membership.html and switch from one premium membership plan to another and any remaining membership credit you have on the current premium membership will be converted and put towards the new premium membership you are changing to.
monthly upgrade?

im not sure you understand. i want one month - and want to know if it will revert back to platinum after that month, or if all the excess credit will dissapear after the upgrade. when i go to buy one month, it says.
Diamond Membership - monthly $12.99
Membership Credit -$47.36
Total $0.00
i just want to pay the extra for this month if possible i dont want it interfering with my platinum

I have the same question. Does anyone know it it is possible to get back to the annual membership after upgrading to a monthly membership?
Perhaps with the 30 day money back guarantee?
I have an annual platinum, but i would like to know if i can do a monthly diamond on top of that, and be able to revert back to platinum when the month is over if i choose to!?