Forgive me for jumping in with yet more unsolicited advice, but I can't help thinking that all your users should be paying at least $5 per month.

An example transition plan:

* switch new users to a free trial -> sign up to $5/month for continued access * observe conversion rate from trial users to paying customers * perform `conversion rate x existing active user base` calculation * send apologetic email to existing users pointing out that free != sustainable, with offer to grandfather in free access to any who feel strongly enough to send you a personal email about it