Allowing apps to run on the home screen (ie: widgets) would definitely be considered a drastic change from where iOS is right now. Apps can't even allow their icons to be updated with information (date, weather etc) so allowing full blown widgets to update in real time would be an overhaul as well as a departure of current app store guidelines.
The first iPhone couldn't run 3rd party apps. Then they allowed them. An overhaul? No, just a new store.
Then they added spotlight. Nothing used to be to the left of page 1. An overhaul? No, just an optional search feature.
Double-clicking used to be user-defined. Now it's a quick-switch menu. An overhaul? No, just shortcuts.
Background APIs? Folders? Widgets? Multiple user profiles?
ALL of these are just usability improvements. They add new features but they don't fundamentally change the way you use the phone.
What would you need to count as an overhaul? Pick at least 1 of the following:
* Direct access to the file system from within iOS.
* Full-screen '1-at-a-time' app interfaces not being the default.
* Removing the 4x4 grid as a way to access apps.
* Treat apps like plugins instead of stand-alone programs (like, Photoshop and Photogene simply modify the photo-viewing app by adding features, they don't stand alone with other apps)
* A hybrid OS where an iPad gains folders, windows, and a mouse cursor when paired with a keyboard and trackpad.
I don't expect any of these in iOS 5, and I don't expect most of them ever.