I lived in country A for a few years, and made a number of purchases on the iTunes store for that country. I have now moved to country B, and want to start using the iTunes store from country B. If I change the country in my account settings, will I lose all my previous purchases, or will I still be able to re-download my purchases from country A from iCloud?
You can create another account in the other country and authorize your iTunes with this both account.
I'm French and I live in Australia and I use 2 different accounts to download music and apps from the both iTunes stores.
No, you will not lose your original iTunes purchases (songs, movies, apps, books, magazines, etc).
However, if you use iTunes on Macs or Windows to manage your apps, you'll only be able to download any app update(s) only if you switch your iTunes login (authenticate by typing in the correct password when prompted) to the same country that you purchased the apps.
If you use iPod Touch, iPhone, or iPad installed with the latest iOS 5, you'll be able to download updates from purchases made with different iTunes country ids. Again, you'll need to authenticate by typing in the correct password when prompted. But unlike iOS 4, you won't need to log out before switching iTunes id.
I am happily using two different iTunes id to buy content from different countries.