You can check episode E1088 of This Week in Startups for some ideas: https://www.youtube.com/watch?v=9OlrMNp90fc&t=1289s
From 4:50 on the founder is taking about their transition from completely free to app with paid features.
Thanks! I actually use Zero myself, so it was fun to watch. Unfortunately, the situations are different, because they left all free features free. In our case, some free features will become paid only.