Can I change the options for the members who already have Instant Email Notofication
 

I have set this: Automatic Thread Subscription Mode to Instant Email Notofication.
But it take a lot of bandwidth to send hundreds of emails every day on every post.

So* now I have change the mode to: Subscribe with no notification.
But that is just for the new registrations.

Can I change the options for the members who already have Instant Email Notofication in their CP??