Although Activision and Infinity Ward announced that Ghosts will have a new next-gen engine, it looks like that isn't the case. Infinity Ward's animation lead, Zach Volker, revealed that the engine isn't completely new and built from scratch, instead it's been "upgraded enough" to be called new. According to Volker it's almost impossible to build and develop a new engine from the ground up in a two year cycle that's imposed.
Zach Volker: "When we're talking about a new engine we're talking about upgrading significant systems within in that engine. We're not talking about throwing it all away and saying we're starting from the ground up."
Volker continued to describe more about how there are obviously going to be remnants of the old Modern Warfare 3 engine here and there, but he believes that they don't need changes, since they wouldn't affect the core gameplay in any way. The team at Infinity Ward wants to only focus on things that that are necessary for the gameplay experience they want, for example, next-gen tech allows for cool 3D smoke that reacts to player movement, but since it doesn't aid the gameplay, they won't pursue that technology.