How about cropping out the offending, inaccurate parts un the AT-AT? That would be a lot easier. Ady, you did well with zoomed in shots on ANH revisited, I recall in the rebel Yavin briefing room during the death star attack you cropped it closer with no noticable graininess. The image above is a bit blurry, but I am sure you can make that scene look good in as you did with those other scenes in ANH-R.

Also, to make another quick comment on that scene, I think it needs to be darkened a bit as it looks too well lit to be the underbelly of the AT-AT, if they is a way you can darken it some it would be great. Maybe let it get brighter for the seconds the saber is lit.