This is a great topic: Inspiring Change.

Two issues seem to be the challenge: 1. overcoming your fear of losing clients; 2. successfully recruiting your client base to adopt a new policy. You might straight away notice that these are two sides of the same issue. The approach is to address your clients as human beings, possibly as colleagues, or just people you know and respect.

I'll try to make my proposal short.
I think you should set a date in which billing of communications-time starts and send a message/letter to all of you existing clients to let them know about it. This broadcast needs to be constructed in a way that is on the one hand informative and on the other conversational, meaning open to discussion. What comes to mind as I write this are a few basic principles. Below you can have a look at them and see if they make sense to you and if you have other ideas.

Inform: Describe in brief the implementation of your new billing policy.
Explain: Let people know what it's all about i.e. background, your intentions, your promise etc.
Invite: Encourage feedback, questions and comments.
Frame: Date of implementation, definitions of billing, deadline for feedback, should all be clear so that a conclusion be reached efficiently and to most people's satisfaction and mostly yours.

Disclaimer: I am a designer with a limited client base. My response here is based mostly on experiences with others and time based insights. Feel free to address my ideas with questions but bare with me if I don't follow up in a time sensitive fashion.

Good luck.