You can change from yearly to monthly billing by going to Settings > Plans and Billing > Change Plan. Then select Monthly at the top and choose your desired plan and credit amount. You can make this change anytime in your billing section.
When you upgrade from yearly to monthly, your cycle resets immediately and you're charged right away for the new monthly plan. This is expected behavior - monthly plans typically cost more per month than the equivalent yearly rate, which is why you're seeing the higher charge.
The new credit allotment gets added on top of your current balance so you can use them right away.
I’m sorry for any confusion caused by the billing change and hope my explanation was clear. Would you like to provide more details about what you’re trying to resolve so I can assist you further?