You can upgrade or downgrade from any subscription plan to any subscription plan at any time inside your Bitrix24 account, here: Subscription > Upgrade > Select the new subscription plan.
When upgrading or downgrading, the amount already paid will be credited towards the new subscription plan purchase and the new subscription cost will be prorated. However, some subscription plans that have been paid for by wire or purchased through a Bitrix reseller may not be prorated (instead, we’ll offer additional days or months of the new subscription plan for the remaining funds). You can contact a Bitrix Sales Department - email@example.com or your original Bitrix Partner (Bitrix Reseller) to have your current subscription plan changed according to your needs.
If you wish to downgrade your commercial subscription to the Free plan, you’ll need to cancel your commercial subscription by disabling the automatic subscription renewal option first. Learn how to cancel your commercial subscription in the article - Subscription Cancellation.