Examples of permission-updating emails are few and far between.

They're what you should use in an ideal world when you do things like change the frequency or content of your email campaigns - to communicate to your list that from now on they're getting something different and if they don't want that then there's an opportunity to opt out.

Design Taxi are doing this currently as they're merging a few lists into one larger campaign.

They've done this by sending a campaign that has a very different visual treatment to their normal campaigns, and have used some quirky but on brand copy to highlight what you'll be missing out on if you unsubscribe.