Can upgrading to iCloud Drive cause loss of shared documents?
Here's my situation: I have a MBP on Mountain Lion, an iPhone 4S on iOS 7, and an iPad 2 on iOS 9. I used iCloud to sync documents and data, primarily for a diary app and mainly with from my iPhone. So when I upgraded my iPad 2, it let me choose between enabling iCloud Drive which would stop syncing from my other devices or to turn off iCloud Drive on the iPad, and I chose the latter. So now I only sync between the Mac and iPhone, and the shared documents have deleted themselves off of the iPad. Pages also won't show iCloud documents anymore either, though the app tells me it's because I need to update it so I'm not sure if it's an iCloud Drive issue there or not.
Now, I'm going to be getting a new iPhone, and it will have iOS 9. I want to restore it from an iTunes backup of my iPhone 4S on iOS 7 to get all my files and apps and settings, but I don't know how to handle it in terms of iCloud Drive.
Regarding the Mac, I am also willing to upgrade to El Capitan which is needed for the new iCloud Drive, by the way.
So I'm thinking, I turn on the new iPhone SE, tell it to set up from an iTunes backup, plug it in to my computer (running Mountain Lion or El Capitan), and then I would assume afterwards it will ask me to turn on iCloud Drive, and I will choose yes, and it will already have the (sizable) contents from my old iPhone's apps' documents and data, and that will delete said data from my old phone and Mac, and then I can upgrade the Mac to El Capitan or choose to turn on iCloud Drive on it at that point, and it will re-download the files from iCloud that it just deleted from the Mac. (And then I can also turn on iCloud Drive on the iPad, assuming that it would put everything from iCloud onto the iPad and not put the iPad's empty folders on to iCloud instead.)
Does all of this sound right? Is it crazy? It's confusing and sounds too easy to me to lose a lot of stuff with the change of system.