For the App Store, it doesn't seem to matter so much. There are 2 scenarios:
- App is available in both countries' stores: People agree that you have to search for the app again, but when you download it, you won't be charged.
- App is NOT available in the new country's store: If the app is on your device, you can update it. If it's in your iTunes library, you can transfer it to your device. If not, you can't get that app back.
For the iTunes Store, it seems more serious. I don't think you can redownload anything at all. At least music has been DRM-free for the past few years, so you can continue to use it, just don't count on iTunes to restore your purchases. I don't know about books/TV/movies.
I haven't tried any of this, it's just what I've noted from people online.
What I wonder is: if you transfer back to your original country, will you get your purchases back? An alternative is to maintain 2 iTunes accounts, but the problem with this is apparently you can only change iTunes accounts on a device once every 90 days or something. I think I'll stick to the same store for now. Looks like most apps are available in all countries anyway.