Was this an artist revision on pixiv? I suspect it's not upscaled as I can see evidence of JPG artifacting.

Yeah should be. The author might accidentally upload original size and replace it later. Since the author is premium member it's doable.

Is there anyway to track on pixiv if an image was replaced?

Used to be.. when a revision exists the link of the image would have a suffix.

But seems not any more.

Edit: I think now you can tell from the link if it doesn't match with the time of post?

Current image (smaller one): http://i3.pixiv.net/img-original/img/2016/04/23/00/23/05/56478826_p0.jpg
Old image (larger one, already deleted): http://i3.pixiv.net/img-original/img/2016/04/23/00/09/00/56478826_p0.jpg

http://www.pixiv.net/member_illust.php?mode=medium&illust_id=56478826

Date in description: 2016年4月23日 00:09