Guys, what?
Of course the 635 is an upgrade.
Hold a 635 beside a 520, you'll see the screen is better. It's brighter, had ClearBlack and has Gorilla Glass 3. The 520 is so washed out by comparison. You're the first person I've heard saying the 520 screen is better, it just isn't. Compare them side by side in a shop. I would doubt that anyone who says the 520 has a better screen has used both phones, the difference in quality is huge.
The specs aren't the same either. I've mentioned Gorilla Glass 3. The 635 has the quad core processor, it's noticeably quicker than the 520, I have a 520 to compare it to. It's snappier. Just look at the benchmarks.
The cameras are both 5mp but the 635, thanks to updated firmware and software, takes better shots. It's no 1020 but it's definitely sharper and more accurate and has less noise in the images.
The 635 is expandable up to 128gb. This is a big plus.
The 635 is nicer to hold and use, it ships with 8.1 and Cyan, it has onscreen buttons, it has LTE 4G.
The biggest plus though is battery life, the 635 has a bigger battery which I am finding to be brilliant, much much better than the 520.
Overall it's an upgrade in every way and I'm laughing at people saying it's not. Go for it.