Let me begin by saying I don't know off the top of my head if Reg CC has any special change-in-terms rules when all you're doing is changing your practice for the new accounts exeption holds that allow you to avoid notifying all of your deposit customers. You should research that question.
However, I would set a date at least 30 days prior to the effective date of your change and send the change in terms to your existing customers. Start providing the new policy to new customers the same day you begin notifying existing customers.
I would treat any account opened prior to date you started providing the disclosures to new customers as if they were under your old practices.
In effect, you'll have two practices in place until all of the old/new customers are out of the pipeline and all you have left is new/new customers.
"If you want to tell people the truth, make them laugh, otherwise they'll kill you." ~ Oscar Wilde