Hearthstone players in Canada, Japan and New Zealand will soon be able to make purchases from Blizzard using their local currencies. The change will be introduced later on this year, and will apply to all Blizzard games.
The only exception to this change is if you also happen to have a recurring World of Warcraft subscription. This will continue to be billed in US Dollars (USD), but new or renewed subscriptions - as well as pre-paid Game Time cards - will use the local currency going forwards.
If you have any funds sitting in your Blizzard Balance, they will be converted from USD to the local currency at the time of the switch-over. The conversion rate will be based on a rate provided by a major exchange on the day of the change.
The stated aim of this currency change is that Blizzard hopes to grow its service globally, while also helping players avoid extra transaction charges that can be incurred when making non-local payments. Additional payment methods - which can't currently be used due to USD restrictions - will also become available in the affected markets.
For a complete guide to the changes, head over to the official Blizzard announcement page. As soon as a date for the transition is announced we'll make a new post about it on the homepage.