Strictly speaking once you change countries you are not supposed to use the iTunes Store in another country.
"The iTunes Service is available to you only in the United States, its territories, and possessions. You agree not to use or attempt to use the iTunes Service from outside these locations. Apple may use technologies to verify your compliance." - http://www.apple.com/legal/itunes/us/terms.html#SERVICE
You are supposed to change the association of your present AppleID to the new country. You will lose access to re-downloads of previously purchased media and to updates for apps. Before you change country make sure you have downloaded and backed up any purchases from your previous country's store. You will not be able to re-download them again once you change countries and it won't be easy to change countries to the previous one once you lose your payment method for the former country (as well as a wait period). As long as you have them on your computer you will be okay.
Change the country of your iTunes Store, App Store, iBooks Store, and Mac App Store account - http://support.apple.com/kb/ht6119 - "After you change the country of your account, you won't see the items you purchased from the previous country’s store in the Purchased section."
Note, if you create a new AppleID as another person recommended it won't help much with your US associated account unless you continue to maintain a US credit card billed to an address in the USA (basically pretend you never moved to Canada and hope Apple isn't tracking the country of your ISP you are using in Canada when you try to do things with the US account). This would add to the complexity of using iTunes (which I won't go into in detail unless you insist) because it isn't really set up for easy multiple account use.