It's the day that Hearthstone fans have been waiting for -- and there sure are a lot of those fans. Today, the Whispers of the Old Gods expansion for Blizzard's collectible card game is live, and it's bringing some major changes to a game that, as of today, is enjoyed by 50 million players on iPhone, Android, Windows and Mac.

Enlarge Image Screenshot by Sean Hollister/CNET

Those changes go above and beyond simply introducing new cards (you can see all 134 new cards here) or making old cards less potent, though both those things are also happening simultaneously. Blizzard's big new gambit, the one that could make Hearthstone even more profitable, is that many older cards will no longer be playable in the game's primary mode.

From now on, year after year, Blizzard will be encouraging those 50 million players to buy new cards to replace their old ones.

As of today, Hearthstone now has two primary game modes: "Standard" and "Wild." In the new "Standard" mode, only the original set of Hearthstone cards and the previous two expansions can be used. "Wild" allows players to use any card but could mean a more limited pool of opponents, which could potentially mean longer wait times between matches.

To ease players into the new mode, Blizzard is offering up to 13 free packs of cards for players who participate this week.

You can read Blizzard's explanation here.