We gave our customer the option to "Opt-in" to have their cancelled checks returned to them in their monthly statements. Prior to this we included all of them in the monthly staements.We now return checks to about 15% of our customers. We would like to discontinue this service completely. Does anyone know, if we need to provide 30 days advance notice to our customers that are still receiving their original checks back or if we need to provide 30 days advance notice to all of our checking customers?

I would think since you want to discontinue the service of returning checks altogether you would need to notify all customer, because as of now they still have the option which you want to eliminate

I agree. Notify everyone. And don't forget to change your disclosure if it includes wording about having the option to get checks returned.
Also, you're right to discontinue the practice of returning checks because it's a huge hassle and with more and more checks getting converted to ACH e-check transactions, it just generates confusion among your customers. But I'm sure you already know that first-hand.