Call of Duty: Ghosts first debuted only a few weeks prior to the launch of the PlayStation 4 and Xbox One. To ease the transition from one console generation to the next, Activision instituted a deal in which those who purchased Call of Duty: Ghosts for either the PlayStation 3 or Xbox 360 could upgrade to the PS4 or Xbox One version without having to purchase the shooter all over again. Details are slightly different on each platform, but the final cost to players is about $10.
All good things must come to an end though, and March 31 will see the closure of this upgrade path. If you still hope to take advantage of the promotion in the little time remaining, have a look at the Call of Duty: Ghosts FAQ, which covers all the details you'll need to upgrade your game.