You can change this for on the “Followers”-page of your feed (how to find it: go to the My Offered Feeds-section where you’ll see a link to the “Followers”-page). Select the users for which you want to change it, then click on “Email delivery channel” in the modify-bar and make the adjustments. 

Above steps change it for existing followers. To make new followers have the right selections from the get-go, please set it as default value on the settings page of your feed (tab “Follow options”). You’ll find a link to the feed’s setting page on your “My Offered Feeds”-page.