You could just let it go for now as Crayola said and then come September let them know that starting for the new year ie Jan 2014 that stats will now be paid days meaning payment is by enrolment. There are only 10 stats throughout the year and if you do it in January then it is two months till the first one they would have to pay if you let Jan 1 go.

But for sure all new families that start will be paying the stats from the day they start.

That is how I raise my fees too in the sense of only for incoming new families and old familes pay the same rate as long as they stay with me.