Monthly (and other limited term) subscriptions going forward, the change is fine, make whatever changes you want, as long as that's what's advertised to people as they make their payment. You begin to advertise this and new subscribers can choose if it is an acceptable deal for them or not. But I paid for a certain amount, you need to provide that amount for the duration of the terms agreed to at the time of the transaction.. which for me, for example, is lifetime.. not less than 4 months that I've been here before it gets cut in half, and future cuts that are sure to come if nothing is done about it now.
