Here's how it works: Once per week, core morpher coupons will appear on the Allods auction house. Only one coupon per class will be available. If you win the auction, you'll be able to grab the coupon out of your in-game mail (for subscribers) or redeem it via the item shop (free players). Once in your inventory, the core morpher coupon enables you to switch to a new class while keeping all of your equipment progress through a system that gives you class-appropriate items that match the rarity and power of the items you currently have.
Webzen advises that you mull the decision carefully, as you may change classes only once per month and you'll be competing with many other players to win your class's weekly auction.