If your site is hosted by us, get in touch with our support team for details about what happens when you cancel your subscription. You can contact our support team through the email address that is given on your site’s admin dashboard.
When you cancel your subscription, your site will be taken offline, so your users will lose access to it. What you can do is cancel your subscription and create a complete backup file that contains all of your site’s data (including its users.) This backup file can be restored to a self-hosted Discourse instance, or to a Discourse instance that’s setup through another Discourse hosting provider. When the backup file is restored on the new site, your users will regain access to the forum.