Not many years ago, there was quite a difference between accounts from different countries when it came to apps and music available for purchase. Nowadays I don't really see that difference, so I wish to change my AppleId's nationality. Is that possible? Will my purchased apps and music be kept with the new nationality?
The App Store purchased items are tied to Apple ID per country.
If you have an Apple ID under registered under email@example.com in US, everything you’ve purchased is only available when you’ve switched to US App Store.
If you use the same Apple ID and switch country to Hong Kong, you won’t be able to download all the apps and musics you’ve purchased in US App Store unless you switch back to US App Store first.
You can, however, download the apps from the US App Store before switching to HK App Store where you can buy an apps from the there separately. The apps bought from HK App Store is only available when your Apple ID’s country has been set in Hong Kong.
For the sake of simplicity, never switch an Apple ID to other regions unless you decide to move to the destination country permanently.
It appears that you can only change the Apple ID's related email address currently. No way available to change the Apple ID nationality unless you create a new Apple account.
Open itunes From the menu bar at the top of your computer screen or at the top of the iTunes window, choose Account > View My Account.
Enter your password, then press the Return or Enter key on your keyboard, or click View Account.
On the Account Information page: To the right of Country/Region, click "Change Country or Region."