The problem with this is that you are not allowing the user to decide which tier they are in, and you'd probably piss a lot of them off by automatically moving

The problem with this is that you are not allowing the user to decide which tier they are in, and you'd probably piss a lot of them off by automatically moving and charging them in the next higher tier. Overage fees are terrible for user retention, because they just irritate people unless it is extremely obvious and normal in your market to charge such fees.