Edit. And maybe re-post the original in scraps for comparison. I did that once It'll be better for your gallery stats that way since scraps don't exactly contribute.
Or, and this might even be better - move the original to scraps without deleting it (so you still have all its faves etc) But submit the new one into your main gallery and just link to the original so people can compare.
Usually I edit my deviations, but this time I'm not sure if people will like the update. The technique is obviously better, but I also changed colors, contrast and overall atmosphere. The old versions looks bright and shiny compared to the new one, which could serve well as a halloween deviation.