Time once again for Covers Redux! Here's how we play the game: I present two Marvel Comics covers side-by side -- the original comic book, plus the reprint book -- and look for changes in the covers. If you see changes I missed, feel free to note them in the comments!
First, we have Amazing Spider-Man #78 and the reprint in Marvel Tales #59. As you can see, the reprint cover clearly is much smaller than the original. The art was scaled down and possibly rotated a little bit, with some background added to it. I can only imagine this was done to accommodate the larger logo for Marvel Tales.
Here's Sgt. Fury #26 and the reprint in #12. As you can see, the image was slightly shifted down and probably shrunk a tiny bit, but the biggest change is adding a bunch of word balloons and bursts!
At first glance, it doesn't seem that there were many changes between Fantastic Four #99 and the reprint in Marvel's Greatest Comics #80... the aft has been shifted down a little bit, probably shrunk as well, but upon closer examination, you'll note that Crystal is missing entirely on the reprint cover, Gorgon has been shifted slightly to the left, Black Bolt has been moved down, and Triton has been moved entirely, with some new background added to replace the explosion from the Torch's fire bolt. I'm not sure why the blurb was changed in the lower right-hand corner, though!
Next, we have Captain America #104 and the reprint in Marvel Super Action #6. It's another case of reducing the original art and shifting it down, with changes to the blurb. Actually, I don't think it was reduced at all, just a lot of cut and pasting of figures to fit in a smaller space!
Next, Amazing Spider-Man #118 and the reprint in Marvel Tales #97. I don't really see any changes there, except that perhaps it was rotated a little bit. Spidey's right leg looks slightly different to my eye.
Finally, here's Sub-Mariner #1 and the reprint in Tales To Astonish (2nd series) #1. Obviously, we're looking at a complete image flip from the original, although fortunately someone made sure Reed Richards' "4" insignia didn't end up reversed!