We have some customers grandfathered in at our earliest pricing tier: $10 per month. When we signed them up at that price, we told them that at some point in the future that would change. I think the important thing early on is to set expectations. Don't pull the rug out from under people, but if your product is 10 days old, tell people that you want the flexibility to change the price in the future.

We're considering adding features not available on prices below X and also offering them a significant discount to switch to annual billing on one of the newer pricing plans.