You can disable the auto-renewal of your subscription at any time by logging into your account. Once logged in, click the circle with your initials or picture in the upper right hand corner. Select Billing and in the "Active Products" section, click Disable Auto-Renew. The advanced features associated with your subscription will continue until the expiration date listed in that box.
Once the subscription expires, any data collected on the sign up(s) involving advanced features (custom questions or hidden field information, for example) will remain and can be viewed by creating a custom report. If you wish to upgrade the account back to a paid plan in the future, you will log into the account and return to Billing. Any previously assigned admins will need to be reinstated using the Tools option on the left side of your account Sign Ups page.