How to migrate your iCloud account I have recently moved from France to the USA.
In the long run, that means I will shut down my French bank account who is becoming useless. But I cannot go on iTunes and change the "bank account" settings or change my address there.

From this article, I believe I will have to buy again all the apps, movies and music I have previously purchased, as well as "forget about" French apps not present in the U.S. store, even the ones for which I wish to keep an account active (not sure how to do that). I believe one cannot "switch" between a US and a French iTunes account at will. I will probably also lose all my Game Center scores, etc. 

1 - How can I measure how much data / apps / settings will be lost?

Also, iCloud documents.

I have several hundred iCloud documents stored in Textedit, Notes, Pages, etcetera, that I absolutely need to work on, on a day-to-day basis.

2 - If there is a simple, step-by-step procedure that would allow me to migrate to a US-based iTunes account with as little loss as possible?

3 - What happens if I move back to France in 5 years? Same losses?

Models of devices: iPhone 4, iOS8, and iMac mid-2011 with Yosemite.

I understand that 1, 2 and 3 are different questions, and if anyone has a problem with this, I will gladly split the question in three different question pages, just let me know.
For #1, one thing to remember is your iCloud account is different from your iTunes/App Store account. You should not lose any of your iCloud documents because that is stored in the iCloud account.

Apple's support page describes the procedure here and you should be able to change it back if you like at a later point. But basically, when you change stores, you don't lose your previous purchases but you cannot update them any longer (because you are no longer hooked up to the store you bought it from in the first place). For these reasons, I have two iTunes/App Store accounts so I can switch back and forth to update my software.