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.
I dislike this immensely if only because as others have said it ruins the immutability of email. But it does already exist.
But sure, what they should be doing with amp is fixing that, not making it worse.