Is there any rule of thumb for weight savings using composite materials. For example, is there an early model large airliner like the B747 or even B737 that later was modified with composite wings or fuselage that shows weight savings?
Above you see similar sized planes with the same 3-class seating (± 1 seat). The newer-more-composite 787 is 16 tonnes heavier, yet delivers up to 20% fuel saving (route dependent). 3.4 of the 16 tonnes are the newer and bigger engines. And the rest is primarily bigger wings (which can be built bigger more easily thanks to composites).
The aim is building bigger wings, not losing weight. The other main goal is the reduced maintenance cost (better fatigue tolerance, no rust, etc.)
The result is an airframe comprising nearly half carbon fiber reinforced plastic and other composites. This approach offers weight savings on average of 20 percent compared to more conventional aluminum designs.
Based on that quotation, the answer is -20% (given the same wing, etc.). Also note the 787 also saves weight from the no-bleed architecture, yet as the above shows, it's not a lighter plane than the one it replaces. (No-bleed also improves engine fuel efficiency, not just weight.)