Because being dated, the demand for the Empire State shrunk.
So the company decided to refit all Skyline Class ships. The Empire State needed it the most
so it was refurbished first in 2015.
In the engineering perspective the ship is now a lot cleaner with being powered by LNG.
Along with that there are new LNG tanks.
When it comes to the interior nearly everything is new.
All cabins, hallways and staircases are redone. Also a larger more spacious stern was added.

Now you have:
-Basketball Court
-A new kindergarden
-Sports and Spa area
-3 Restaurants
-3 Lounges
-3 Bars
-1 Café
-Liberty Theatre
-Library

Enjoy :)