Ask HN: SaaS going from free to paid. How to transition existing users?

Long story short, our SaaS [0] has been free for a few years. We're now transitioning into paid plans.

We've started charging new users and the conversion rate has been good. Now the question is, how do we transition our existing free users? There is a chance some people are gonna get pissed (even though they've been using our service for free).

Are there any best practices on what to do in such a situation? The first thing that comes to mind is to offer a discount on the yearly plan. Anything else?

[0] - https://lookastic.com