Airbus want to stick some slighly more efficent engines on their A32x
The 15% fuel savings are cumulative with some other aerodynamic changes as well, such as the new blended winglets or sharklets as they're being called which offer a 3.5% fuel burn reduction.
The sharklets as implemented on the NEO cannot be retrofitted, however Airbus have just announced that they are looking at alternatives for retrofit.