No billing contracts here. You can upgrade or downgrade whenever you like from Billing & Payments (click your name in the top-left corner, then Billing & Payments).

Upgrades take effect immediately. You are billed for the new plan right away, receive its monthly credits, and a new billing cycle starts from that day. So if you previously renewed on the 1st and upgrade on the 10th, your billing date becomes the 10th.

Downgrades are scheduled rather than immediate: your current plan stays active until your next billing date, and the new plan starts from then. You will see the scheduled change reflected in Billing & Payments, and you can cancel the scheduled change any time before it takes effect.

Your existing credit balance carries over when you change between paid plans. Monthly credits from the new plan are added at the start of its first cycle.