As someone who's spent the last 2 months porting code from iOS6 to iOS7, I disagree. The API changes (documented or otherwise...) are large enough that this really was more of a port than it was a simple update.
It may look like simple rearrangement (I disagree with that assessment, but whatever), but under the hood a great deal has changed.
Quick examples:
- The entire way iOS handles text rendering has changed (makes sense, CoreText), so methods that calculate text rendering now return different results than they did before. More crucially, text measurements can now return non-round numbers, so if you're using the results without rounding them off, you will get pixel alignment problems galore. Previous versions of iOS rounded off results before returning them to you (though this is not guaranteed via documentation, or even addressed).
- Text rendering has also changed, so even if you're not using the system fonts, the exact same font will render with subtly different widths and heights. In densely packed UIs this can be problematic.
- UITableViewCell's underlying implementation has changed. More specifically, backgroundView is now by default a plain white view, where it was nil before. If you have table views that are transparent, surprise, they are now all white. There are also some subtle differences that can cause trouble if you've heavily customized UITableView - I suspect personally that UITableView now has a UICollectionView (or UICollectionView-esque) based implementation (it seemed entirely separate before), since some of the behavior now aligns.
For instance, we already had custom text metrics code since our app's font is italic, and that code already rounded to non-retina boundaries. And I think nearly all our tableviews already had custom backgroundViews.
Our biggest incompatibility was a progress HUD popup that subclassed UIAlertView, and that drawing broke completely in iOS 7 (it just turned into a little blurry white bar in the middle of the screen.
- You wouldn't expect Apple to change core apps like Phone dramatically. It's a very mature feature that isn't going anywhere and is practically identical on every smartphone. You get a numpad, you get your contacts list, your voicemail, etc. All of which has been given a new coat of paint, but fundamentally hasn't changed much.
- You see deeper changes in the built-in apps that aren't quite as mature/core. There is for example a large change in UX in iOS to move away from hard, distracting transitions towards more subtle transitions that imply information hierarchy. For example if you tap on a date in a calendar, instead of hard-wiping you to another piece of UI altogether to show you appointments, it dynamically expands the day-view from the date itself. If you tap on an album in Photos it will dynamically expand that album to fill the screen with additional photos. All of this goes towards better educating the user on information hierarchy and is much, much less jarring than before. As a third party dev that's been working with Apple on iOS7 prep, I know for a fact that they're pushing this new UX hard - you will see more 3rd party apps go towards this model in the coming months. You see some uses of this in first-party apps, but users will notice most when 3rd parties start adopting this.
- Many of the most substantial changes are under the hood and aren't being (visibly or ostentatiously) used in the stock apps, mostly because there really isn't a place for them. They will, however, power a new generation of apps in the coming months - things like a new backgrounding model will let apps do more while not in use, and also in some types of apps greatly reduce the loading/refresh time when you return to them. Or even more subtle things like dynamics, which will make scrolling feel very different in apps.
In short, users will feel some of these changes immediately, but ultimately the deep changes in iOS won't be felt until third party dev adoption is there.