I made my web app side project (a text editor for Dropbox) into a paid product. Once traffic picked up on the original (free) app and I gathered feature request

I made my web app side project (a text editor for Dropbox) into a paid product. Once traffic picked up on the original (free) app and I gathered feature requests from users, I rewrote the app from scratch and charged for that while keeping the old one online. I did this about 1 year after launching the original app. It was a great decision for me and for my users because I was able to implement new features and cover their costs at a price that users were happy to pay. In my case it helped that user data was all in Dropbox, so there wouldn't be any data migration to do in case they decided to move off of my app. I've also found paying customers to be really appreciative and grateful for the app.