Aftershock is available for purchase on Steam, and will be available on GOG and the 3D Realms store later this week. It's planned to release on consoles eventually, but no date has been announced for that.

As with any new expansion pack or DLC, it's going to result in some leaderboard changes. The existing categories will stay the same, but new categories will be added for Arrange Mode and Aftershock's campaign shortly. Individual levels will be added, too.