It's been done, but I wanted to take my own comparison pics. Interestingly, the rubber parts on my MKIII haven't really changed that much in color. These photos are unmolested so what you see in these photos is the difference in color if you look at in person. I have seen some photos that make the two pieces look drastically different.
I was trying to show how much thicker the MKIV torso is from front to back than the mkIII and I think I kind of failed. The MKIV is a much bulkier figure in general (this is good) something that I haven't really been able to tell from seeing photos (including my own here).
The new one is clearly more detailed and realistic though neither likeness is particularly great. What this really shows off to me is how much better the proportions are on the MKIV. The MKIII doesn't even look like it's human.