Ask HN: Increasing your prices as you scale services

Hi guys, about to launch a freemium model of my currently free product. We will probably want to raise our pricing about 6 months after launching the first paid version, as our product offering becomes significantly richer. Anybody have experience with this? My initial thoughts were just to grandfather existing users at the old rates, and have the rate increase only apply to new users, but would love to hear from anyone with experience.