If your account is billed monthly, you may upgrade, downgrade or cancel at any time. Once you have downgraded out of a paid plan, your Forms will stay in your Salesforce Org as long as you do not uninstall the application.
To downgrade your account, please submit an email and our Support Team will direct your case accordingly.
Note: At this time, Salesforce Application users must submit an email. Formstack.com is considered a separate product, and thus you will not be able to log in there to downgrade your account.