Ask HN: How do I get a customer to upgrade their software?

My last post talked about a SAAS idea. I have already implemented it before for a client. I've now spoken to this client and received their valuable feedback about the application. Overall, they really like it and it helps their business be more efficient.

I want to try and convert them to a paid monthly subscription model. The advantages of this for them is: new features, bug fixes, don't have to worry about hosting, etc. Should I offer them a free trial to get my first customer?