Any email with an img tag can change its contents between views. AMP is not required to do that. The img content is retrieved from a server (or a proxy) every time the email is opened, and that content can change between opens.
To add to this, citi recently starting using this to notify you when you reached your spending bonus after signing up for a new card. I've seen it used to load gifs that are countdown timers that once the final time is reached will always display 00:00:00.
I dislike this immensely if only because as others have said it ruins the immutability of email. But it does already exist.