When you downgrade your account the changes will apply at the end of the current billing cycle. You can still use your subscription to send out documents for signature until then.
For example, you have an annual plan that was purchased on January 1, 2020. You decide to switch to monthly payments on May 30, 2020. The changes to your plan will apply on January 1, 2021, and you can still use your remaining limit of document folio sends until then. The new limits apply on the date the plan changes.