Grandfathering is the best thing to do here.

A good trick I've heard is to launch at a high price - say $100 per month - and do promos through marketing channels where you are quite liberal with discounts. This gives the perception of high value on a limited time offer while still making it clear what your price point is.

You then make the discounts more and more scarce over time and for smaller and smaller amounts until you settle on optimal pricing.