Our first selection today compares the covers to X-Men #20 and #71! As you can see, this is another situation where the original cover was enlarged slightly for the reprint, with some slight shifting to the right, I believe. It's also been shifted up!
Next we have Tales to Astonish #99 and Marvel Super-Heroes #54. As you can see, we're dealing with the issue of the trade dress Marvel was using here (although this isn't as bad as when the entire cover art was boxed in). The original cover was fine, but wouldn't fit here too well, would it? At least we got what appears to be a new Jack Kirby cover out of it, although I'm not sure how this fits in with Kirby's departures from Marvel!
Looking at Amazing Spider-Man #56 and the new cover commissioned for the reprint in Marvel Tales #41, you'd hardly think they were for the same story, would you? I love the original cover, but come on, I can't complain about a Gil Kane cover, can I?
Lots of cut-and-paste happening when Fantastic Four #67 was reprinted in Marvel's Greatest Comics #50. Most of the surrounding characters around Him's cocoon were moved (most significantly the Torch), with the blurbs moving as well.
Next, it's X-Men #15 and #68! Looks like things were enlarged for the reprint, as well as the addition of word balloons for Professor X! Note that the background is now black, with the energy around the Professor's mental projection being altered slightly, too. Some other background elements were changed as well! Actually, according to the Grand Comic Book Database, this is actually the original cover, with the "original" cover being altered prior to publication. A stat of the unaltered cover was used for the reprint! Crazy, eh?
You'll have to find any changes made between Sgt. Fury #28 and #91, 'cause I can't find any!
The cover art for Avengers #60 was reduced a fair amount for the reprint in Marvel Super Action #21! It's all been shifted to the right, to boot, although new art was added on both sides.