I am not sure how it works exactly, or what it's used for per se, but it's happened to me twice now (rather than just being deleted) and I was curious how someone does it, or what circumstances it is used for?

I'd also like to possibly do it myself for something

what kemo said. if you want to replace an image with a superior version but you really want to keep the comments and upvotes the same, then you'd have to ask someone who's got the replacements beta (mostly privileged users+ if i'm not mistaken)

otherwise you'd have to make a whole new post with the superior version and request the inferior version to be deleted (though the post itself will be preserved and can still be visited, comments and all, just no art)