When switching from a free to a paid service, start off by giving your clients plenty of notice. This will give them time to submit feedback. I might be stating the obvious here but be sure to price it fairly. Consider offering a way for your customers to get your product for free. For example, offer a customer that gets other members to join the payment model, a period of free use. Additional strategies also include providing discounted pricing for long-time customers or giving them exclusive benefits. Last but not least, try and make the transition gradual if possible.