Yes, upgrading or downgrading your plan is possible on a monthly subscription.
All you need to do is choose your new plan and the system will automatically update your account. This is available by navigating to My Account > Subscription (top right Menu), then moving the slider to your preferred plan and click on the "Choose Plan" blue button.
Here is a screenshot that shows this action:
Note: Downgraded accounts will not lose their credits, but any changes will not take place until the end of the current billing cycle.