Will I lose previous purchases when I change country in iTunes
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.