I have two recommendations:
Experiment with your new payment model as a "Special Offer" and see how many new clients choose it. You could A/B test it if you get enough search volumes or just swap it out for a month and see if your conversion rates change significantly.

Secondly, Offer the new lower monthly fee to existing clients if they successfully introduce a new client. Give them a pre-written message to share out on their social media accounts.

So a strong offer, metrics to track outcomes and a great bit of copywriting [which we can do] backed with proactive direct marketing will get you the information to make your decision.