If you downgrade your plan from Yearly to a Monthly subscription (only possible while staying in the same tier), we will credit your account with the remaining "balance" on your plan, i.e. the amount that has been unused.
If your credit is high enough, during the following months, we will use that credit to pay for the subscription and will not charge your card. Once the credit is finished, or insufficient, then we will start charging you again.
The amount of credit remaining can be found on your invoice, which are available on the Billing submenu of the General Settings section.